From 96a3f2db0147c9596c37825924fa3fd22fc59377 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 11 Dec 2024 11:34:54 +0000
Subject: [PATCH 01/48] Init commit, part way through ensuring Nones aren't
 wrapped in np.array

---
 lib/iris/_data_manager.py | 44 +++++++++++++++++++++++++++++++++------
 lib/iris/cube.py          |  5 +++--
 2 files changed, 41 insertions(+), 8 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index dbd122ba04..52809f2ace 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -6,6 +6,9 @@
 
 import copy
 
+import iris.exceptions
+import iris.warnings
+from warnings import warn
 import numpy as np
 import numpy.ma as ma
 
@@ -15,7 +18,7 @@
 class DataManager:
     """Provides a well defined API for management of real or lazy data."""
 
-    def __init__(self, data):
+    def __init__(self, data, shape=None):
         """Create a data manager for the specified data.
 
         Parameters
@@ -35,6 +38,20 @@ def __init__(self, data):
 
         # Enforce the manager contract.
         self._assert_axioms()
+        # if cube is empty
+        if (shape is None) and (data is not None):
+            self.data = data
+            self._shape = None
+        # if cube is dataless
+        elif (shape is not None) and (data is None):
+            self._shape = shape
+        elif (shape is not None) and (data is not None):
+            msg = f"A cube may not be created with both data and a custom shape."
+            raise iris.exceptions.InvalidCubeError(msg)
+        else:
+            msg = f"A cube may not be created without both data and a custom shape."
+            warn(msg, iris.warnings.IrisUserWarning)
+
 
     def __copy__(self):
         """Forbid :class:`~iris._data_manager.DataManager` instance shallow-copy support."""
@@ -128,6 +145,7 @@ def _assert_axioms(self):
         # Ensure there is a valid data state.
         is_lazy = self._lazy_array is not None
         is_real = self._real_array is not None
+        has_shape = self._shape is not None
         emsg = "Unexpected data state, got {}lazy and {}real data."
         state = is_lazy ^ is_real
         assert state, emsg.format("" if is_lazy else "no ", "" if is_real else "no ")
@@ -148,6 +166,7 @@ def _deepcopy(self, memo, data=None):
         :class:`~iris._data_manager.DataManager` instance.
 
         """
+        # @TODO how to ask copy to make an empty cube, special value? flag?
         try:
             if data is None:
                 # Copy the managed data.
@@ -221,17 +240,22 @@ def data(self, data):
         """
         # Ensure we have numpy-like data.
         if not (hasattr(data, "shape") and hasattr(data, "dtype")):
-            data = np.asanyarray(data)
+            # data = np.asanyarray(data)
+            if data is not None:
+                data = np.asanyarray(data)
 
         # Determine whether the class instance has been created,
         # as this method is called from within the __init__.
         init_done = self._lazy_array is not None or self._real_array is not None
+        # @TODO set self._shape every time you change the data
 
         if init_done and self.shape != data.shape:
             # The _ONLY_ data reshape permitted is converting a 0-dimensional
             # array i.e. self.shape == () into a 1-dimensional array of length
             # one i.e. data.shape == (1,)
-            if self.shape or data.shape != (1,):
+            if (not is_lazy_data(data)) and data is None:
+                self._shape = self.shape
+            elif self.shape or data.shape != (1,):
                 emsg = "Require data with shape {!r}, got {!r}."
                 raise ValueError(emsg.format(self.shape, data.shape))
 
@@ -242,7 +266,8 @@ def data(self, data):
         else:
             if not ma.isMaskedArray(data):
                 # Coerce input data to ndarray (including ndarray subclasses).
-                data = np.asarray(data)
+                if data is not None:
+                    data = np.asarray(data)
             if isinstance(data, ma.core.MaskedConstant):
                 # Promote to a masked array so that the fill-value is
                 # writeable to the data owner.
@@ -261,12 +286,19 @@ def dtype(self):
     @property
     def ndim(self):
         """The number of dimensions covered by the data being managed."""
-        return self.core_data().ndim
+        return len(self.shape)
 
     @property
     def shape(self):
         """The shape of the data being managed."""
-        return self.core_data().shape
+        print("1", self.data)
+        if self.data is None:
+            print("2", self.data)
+        # if self._lazy_array is None and np.all(self.data == None):
+            result = self._shape
+        else:
+            result = self.core_data().shape
+        return result
 
     def copy(self, data=None):
         """Return a deep copy of this :class:`~iris._data_manager.DataManager` instance.
diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index 30ac3432b7..abeceff819 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -1190,7 +1190,8 @@ def _walk_nodes(node):
 
     def __init__(
         self,
-        data: np.typing.ArrayLike,
+        data: np.typing.ArrayLike, # should this now be none? test this
+        shape: tuple | None = None,
         standard_name: str | None = None,
         long_name: str | None = None,
         var_name: str | None = None,
@@ -1276,7 +1277,7 @@ def __init__(
         self._metadata_manager = metadata_manager_factory(CubeMetadata)
 
         # Initialise the cube data manager.
-        self._data_manager = DataManager(data)
+        self._data_manager = DataManager(data, shape)
 
         #: The "standard name" for the Cube's phenomenon.
         self.standard_name = standard_name

From 7114f0f8c58c88e56de877db081f57a9804b43eb Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 11 Dec 2024 11:38:12 +0000
Subject: [PATCH 02/48] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 lib/iris/_data_manager.py | 9 ++++-----
 lib/iris/cube.py          | 2 +-
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 52809f2ace..12dff15512 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -5,14 +5,14 @@
 """Management of common state and behaviour for cube and coordinate data."""
 
 import copy
-
-import iris.exceptions
-import iris.warnings
 from warnings import warn
+
 import numpy as np
 import numpy.ma as ma
 
 from iris._lazy_data import as_concrete_data, as_lazy_data, is_lazy_data
+import iris.exceptions
+import iris.warnings
 
 
 class DataManager:
@@ -52,7 +52,6 @@ def __init__(self, data, shape=None):
             msg = f"A cube may not be created without both data and a custom shape."
             warn(msg, iris.warnings.IrisUserWarning)
 
-
     def __copy__(self):
         """Forbid :class:`~iris._data_manager.DataManager` instance shallow-copy support."""
         name = type(self).__name__
@@ -294,7 +293,7 @@ def shape(self):
         print("1", self.data)
         if self.data is None:
             print("2", self.data)
-        # if self._lazy_array is None and np.all(self.data == None):
+            # if self._lazy_array is None and np.all(self.data == None):
             result = self._shape
         else:
             result = self.core_data().shape
diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index abeceff819..66f694da8d 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -1190,7 +1190,7 @@ def _walk_nodes(node):
 
     def __init__(
         self,
-        data: np.typing.ArrayLike, # should this now be none? test this
+        data: np.typing.ArrayLike,  # should this now be none? test this
         shape: tuple | None = None,
         standard_name: str | None = None,
         long_name: str | None = None,

From 84ea908e91a529e8c42e00c9b85e9c85879dcc64 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 11 Dec 2024 12:17:59 +0000
Subject: [PATCH 03/48] None types are no longer wrapped

---
 lib/iris/_data_manager.py | 32 ++++++++++++++------------------
 1 file changed, 14 insertions(+), 18 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 52809f2ace..1e7a5171b3 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -32,6 +32,7 @@ def __init__(self, data, shape=None):
         # Initialise the instance.
         self._lazy_array = None
         self._real_array = None
+        self._shape = shape
 
         # Assign the data payload to be managed.
         self.data = data
@@ -39,20 +40,15 @@ def __init__(self, data, shape=None):
         # Enforce the manager contract.
         self._assert_axioms()
         # if cube is empty
-        if (shape is None) and (data is not None):
-            self.data = data
-            self._shape = None
-        # if cube is dataless
-        elif (shape is not None) and (data is None):
-            self._shape = shape
-        elif (shape is not None) and (data is not None):
+        if (shape is not None) and (data is not None):
             msg = f"A cube may not be created with both data and a custom shape."
             raise iris.exceptions.InvalidCubeError(msg)
-        else:
+        elif (shape is None) and (data is None):
             msg = f"A cube may not be created without both data and a custom shape."
             warn(msg, iris.warnings.IrisUserWarning)
 
 
+
     def __copy__(self):
         """Forbid :class:`~iris._data_manager.DataManager` instance shallow-copy support."""
         name = type(self).__name__
@@ -145,10 +141,12 @@ def _assert_axioms(self):
         # Ensure there is a valid data state.
         is_lazy = self._lazy_array is not None
         is_real = self._real_array is not None
-        has_shape = self._shape is not None
+        is_dataless = not(is_lazy or is_real) and self._shape is not None # if I remove the second check, allows empty arrays, like old behaviour
         emsg = "Unexpected data state, got {}lazy and {}real data."
-        state = is_lazy ^ is_real
-        assert state, emsg.format("" if is_lazy else "no ", "" if is_real else "no ")
+        state = (is_lazy ^ is_real) or is_dataless
+        if not state:
+            raise iris.exceptions.InvalidCubeError(emsg.format("" if is_lazy else "no ", "" if is_real else "no "))
+
 
     def _deepcopy(self, memo, data=None):
         """Perform a deepcopy of the :class:`~iris._data_manager.DataManager` instance.
@@ -239,21 +237,21 @@ def data(self, data):
 
         """
         # Ensure we have numpy-like data.
+        dataless = data is None
         if not (hasattr(data, "shape") and hasattr(data, "dtype")):
             # data = np.asanyarray(data)
-            if data is not None:
+            if not dataless:
                 data = np.asanyarray(data)
 
         # Determine whether the class instance has been created,
         # as this method is called from within the __init__.
         init_done = self._lazy_array is not None or self._real_array is not None
-        # @TODO set self._shape every time you change the data
 
-        if init_done and self.shape != data.shape:
+        if init_done and not dataless and self.shape != data.shape:
             # The _ONLY_ data reshape permitted is converting a 0-dimensional
             # array i.e. self.shape == () into a 1-dimensional array of length
             # one i.e. data.shape == (1,)
-            if (not is_lazy_data(data)) and data is None:
+            if (not is_lazy_data(data)) and dataless:
                 self._shape = self.shape
             elif self.shape or data.shape != (1,):
                 emsg = "Require data with shape {!r}, got {!r}."
@@ -266,7 +264,7 @@ def data(self, data):
         else:
             if not ma.isMaskedArray(data):
                 # Coerce input data to ndarray (including ndarray subclasses).
-                if data is not None:
+                if not dataless:
                     data = np.asarray(data)
             if isinstance(data, ma.core.MaskedConstant):
                 # Promote to a masked array so that the fill-value is
@@ -291,9 +289,7 @@ def ndim(self):
     @property
     def shape(self):
         """The shape of the data being managed."""
-        print("1", self.data)
         if self.data is None:
-            print("2", self.data)
         # if self._lazy_array is None and np.all(self.data == None):
             result = self._shape
         else:

From cdc51d5f4216243ef7cd572ff4827edbf18d48f3 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 11 Dec 2024 12:22:32 +0000
Subject: [PATCH 04/48] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 lib/iris/_data_manager.py | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 87caf0738d..5200d57a93 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -139,12 +139,15 @@ def _assert_axioms(self):
         # Ensure there is a valid data state.
         is_lazy = self._lazy_array is not None
         is_real = self._real_array is not None
-        is_dataless = not(is_lazy or is_real) and self._shape is not None # if I remove the second check, allows empty arrays, like old behaviour
+        is_dataless = (
+            not (is_lazy or is_real) and self._shape is not None
+        )  # if I remove the second check, allows empty arrays, like old behaviour
         emsg = "Unexpected data state, got {}lazy and {}real data."
         state = (is_lazy ^ is_real) or is_dataless
         if not state:
-            raise iris.exceptions.InvalidCubeError(emsg.format("" if is_lazy else "no ", "" if is_real else "no "))
-
+            raise iris.exceptions.InvalidCubeError(
+                emsg.format("" if is_lazy else "no ", "" if is_real else "no ")
+            )
 
     def _deepcopy(self, memo, data=None):
         """Perform a deepcopy of the :class:`~iris._data_manager.DataManager` instance.

From 7b402e03e3aa7496aae1293d649b9ba384f11027 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 11 Dec 2024 14:00:47 +0000
Subject: [PATCH 05/48] clarified axiom check

---
 lib/iris/_data_manager.py | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 87caf0738d..2073875f30 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -41,10 +41,10 @@ def __init__(self, data, shape=None):
         self._assert_axioms()
         # if cube is empty
         if (shape is not None) and (data is not None):
-            msg = f"A cube may not be created with both data and a custom shape."
+            msg = "A cube may not be created with both data and a custom shape."
             raise iris.exceptions.InvalidCubeError(msg)
         elif (shape is None) and (data is None):
-            msg = f"A cube may not be created without both data and a custom shape."
+            msg = "A cube may not be created without both data and a custom shape."
             warn(msg, iris.warnings.IrisUserWarning)
 
     def __copy__(self):
@@ -137,13 +137,14 @@ def __repr__(self):
     def _assert_axioms(self):
         """Definition of the manager state, that should never be violated."""
         # Ensure there is a valid data state.
-        is_lazy = self._lazy_array is not None
-        is_real = self._real_array is not None
-        is_dataless = not(is_lazy or is_real) and self._shape is not None # if I remove the second check, allows empty arrays, like old behaviour
-        emsg = "Unexpected data state, got {}lazy and {}real data."
-        state = (is_lazy ^ is_real) or is_dataless
-        if not state:
-            raise iris.exceptions.InvalidCubeError(emsg.format("" if is_lazy else "no ", "" if is_real else "no "))
+        empty = self._lazy_array is None and self._real_array is None
+        overfilled = self._lazy_array is not None and self._real_array is not None
+        if overfilled:
+            msg = "Unexpected data state, got both lazy and real data."
+            raise iris.exceptions.InvalidCubeError(msg)
+        elif empty and self._shape is None:  # if I remove the second check, allows empty arrays, like old behaviour
+            msg = "Unexpected data state, got no lazy or real data, and no shape."
+            raise iris.exceptions.InvalidCubeError(msg)
 
 
     def _deepcopy(self, memo, data=None):

From bc1ee6f8c69a700a606bfe6f43d8ffa26149f05a Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 11 Dec 2024 16:07:38 +0000
Subject: [PATCH 06/48] moved shape order in Cube

---
 lib/iris/_data_manager.py | 7 +------
 lib/iris/cube.py          | 4 ++--
 2 files changed, 3 insertions(+), 8 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 2073875f30..e1d55c53d8 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -32,20 +32,15 @@ def __init__(self, data, shape=None):
         # Initialise the instance.
         self._lazy_array = None
         self._real_array = None
-        self._shape = shape
 
         # Assign the data payload to be managed.
         self.data = data
+        self._shape = shape
 
-        # Enforce the manager contract.
-        self._assert_axioms()
         # if cube is empty
         if (shape is not None) and (data is not None):
             msg = "A cube may not be created with both data and a custom shape."
             raise iris.exceptions.InvalidCubeError(msg)
-        elif (shape is None) and (data is None):
-            msg = "A cube may not be created without both data and a custom shape."
-            warn(msg, iris.warnings.IrisUserWarning)
 
     def __copy__(self):
         """Forbid :class:`~iris._data_manager.DataManager` instance shallow-copy support."""
diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index 66f694da8d..074b42197b 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -1190,8 +1190,7 @@ def _walk_nodes(node):
 
     def __init__(
         self,
-        data: np.typing.ArrayLike,  # should this now be none? test this
-        shape: tuple | None = None,
+        data: np.typing.ArrayLike,
         standard_name: str | None = None,
         long_name: str | None = None,
         var_name: str | None = None,
@@ -1205,6 +1204,7 @@ def __init__(
         cell_measures_and_dims: Iterable[tuple[CellMeasure, int]] | None = None,
         ancillary_variables_and_dims: Iterable[tuple[AncillaryVariable, int]]
         | None = None,
+        shape: tuple | None = None,
     ):
         """Create a cube with data and optional metadata.
 

From 16779aa93ff89fa7a7fa5f3209c61595cd283fc2 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 11 Dec 2024 16:09:49 +0000
Subject: [PATCH 07/48] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 lib/iris/_data_manager.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index e1d55c53d8..3e39e3f2c6 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -137,11 +137,12 @@ def _assert_axioms(self):
         if overfilled:
             msg = "Unexpected data state, got both lazy and real data."
             raise iris.exceptions.InvalidCubeError(msg)
-        elif empty and self._shape is None:  # if I remove the second check, allows empty arrays, like old behaviour
+        elif (
+            empty and self._shape is None
+        ):  # if I remove the second check, allows empty arrays, like old behaviour
             msg = "Unexpected data state, got no lazy or real data, and no shape."
             raise iris.exceptions.InvalidCubeError(msg)
 
-
     def _deepcopy(self, memo, data=None):
         """Perform a deepcopy of the :class:`~iris._data_manager.DataManager` instance.
 

From f42c19d9918f27f007a568bb130277d1cad9a4b2 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 11 Dec 2024 16:57:17 +0000
Subject: [PATCH 08/48] replace call to self.data with self.core_data()

---
 lib/iris/_data_manager.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index e1d55c53d8..5dc516d411 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -34,10 +34,11 @@ def __init__(self, data, shape=None):
         self._real_array = None
 
         # Assign the data payload to be managed.
-        self.data = data
         self._shape = shape
+        self.data = data
+
 
-        # if cube is empty
+        # if cube has shape and data
         if (shape is not None) and (data is not None):
             msg = "A cube may not be created with both data and a custom shape."
             raise iris.exceptions.InvalidCubeError(msg)
@@ -233,7 +234,6 @@ def data(self, data):
         # Ensure we have numpy-like data.
         dataless = data is None
         if not (hasattr(data, "shape") and hasattr(data, "dtype")):
-            # data = np.asanyarray(data)
             if not dataless:
                 data = np.asanyarray(data)
 
@@ -283,7 +283,7 @@ def ndim(self):
     @property
     def shape(self):
         """The shape of the data being managed."""
-        if self.data is None:
+        if self.core_data() is None:
             result = self._shape
         else:
             result = self.core_data().shape

From 6d62c7d6133615a402b3eb2d45d2119c3c3f1645 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 11 Dec 2024 16:58:15 +0000
Subject: [PATCH 09/48] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 lib/iris/_data_manager.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 2731ba2c3f..c5f62b1966 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -37,7 +37,6 @@ def __init__(self, data, shape=None):
         self._shape = shape
         self.data = data
 
-
         # if cube has shape and data
         if (shape is not None) and (data is not None):
             msg = "A cube may not be created with both data and a custom shape."

From cc13e6d9156790d3ab70df267c7bdb1a966dea29 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 11 Dec 2024 17:14:10 +0000
Subject: [PATCH 10/48] fixed test regex

---
 lib/iris/_data_manager.py                            |  1 -
 lib/iris/tests/unit/data_manager/test_DataManager.py | 10 +++++-----
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index c5f62b1966..1ae00798a3 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -5,7 +5,6 @@
 """Management of common state and behaviour for cube and coordinate data."""
 
 import copy
-from warnings import warn
 
 import numpy as np
 import numpy.ma as ma
diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index b419e556a7..431ea615c4 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -3,7 +3,7 @@
 # This file is part of Iris and is released under the BSD license.
 # See LICENSE in the root of the repository for full licensing details.
 """Unit tests for the :class:`iris._data_manager.DataManager`."""
-
+import iris.exceptions
 # Import iris.tests first so that some things can be initialised before
 # importing anything else.
 import iris.tests as tests  # isort:skip
@@ -167,14 +167,14 @@ def setUp(self):
 
     def test_array_none(self):
         self.dm._real_array = None
-        emsg = "Unexpected data state, got no lazy and no real data"
-        with self.assertRaisesRegex(AssertionError, emsg):
+        emsg = "Unexpected data state, got no lazy or real data, and no shape."
+        with self.assertRaisesRegex(iris.exceptions.InvalidCubeError, emsg):
             self.dm._assert_axioms()
 
     def test_array_all(self):
         self.dm._lazy_array = self.lazy_array
-        emsg = "Unexpected data state, got lazy and real data"
-        with self.assertRaisesRegex(AssertionError, emsg):
+        emsg = "Unexpected data state, got both lazy and real data."
+        with self.assertRaisesRegex(iris.exceptions.InvalidCubeError, emsg):
             self.dm._assert_axioms()
 
 

From 71c7ae8eb6c5af40350a673c4f00e9f7cc8960c8 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 11 Dec 2024 17:16:25 +0000
Subject: [PATCH 11/48] precommit

---
 lib/iris/tests/unit/data_manager/test_DataManager.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 431ea615c4..c8201726ba 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -3,7 +3,9 @@
 # This file is part of Iris and is released under the BSD license.
 # See LICENSE in the root of the repository for full licensing details.
 """Unit tests for the :class:`iris._data_manager.DataManager`."""
+
 import iris.exceptions
+
 # Import iris.tests first so that some things can be initialised before
 # importing anything else.
 import iris.tests as tests  # isort:skip

From e59d5c9652c0c16174dfdbbd8d3415fb2330b7ed Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Thu, 12 Dec 2024 11:16:33 +0000
Subject: [PATCH 12/48] written tests, and refactored redundant checks

---
 lib/iris/_data_manager.py                     | 32 ++++++-------
 .../unit/data_manager/test_DataManager.py     | 47 +++++++++++++++++++
 2 files changed, 63 insertions(+), 16 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 1ae00798a3..35677ddac6 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -28,6 +28,10 @@ def __init__(self, data, shape=None):
             managed.
 
         """
+        if (shape is not None) and (data is not None):
+            msg = "A cube may not be created with both data and a custom shape."
+            raise iris.exceptions.InvalidCubeError(msg)
+
         # Initialise the instance.
         self._lazy_array = None
         self._real_array = None
@@ -36,11 +40,6 @@ def __init__(self, data, shape=None):
         self._shape = shape
         self.data = data
 
-        # if cube has shape and data
-        if (shape is not None) and (data is not None):
-            msg = "A cube may not be created with both data and a custom shape."
-            raise iris.exceptions.InvalidCubeError(msg)
-
     def __copy__(self):
         """Forbid :class:`~iris._data_manager.DataManager` instance shallow-copy support."""
         name = type(self).__name__
@@ -230,23 +229,24 @@ def data(self, data):
             managed.
 
         """
-        # Ensure we have numpy-like data.
+        # If data is None, ensure previous shape is maintained, and that it is
+        # not wrapped in an np.array
         dataless = data is None
-        if not (hasattr(data, "shape") and hasattr(data, "dtype")):
-            if not dataless:
-                data = np.asanyarray(data)
+        if dataless:
+            self._shape = self.shape
 
-        # Determine whether the class instance has been created,
-        # as this method is called from within the __init__.
-        init_done = self._lazy_array is not None or self._real_array is not None
+        # Ensure we have numpy-like data.
+        elif not (hasattr(data, "shape") and hasattr(data, "dtype")):
+            data = np.asanyarray(data)
 
-        if init_done and not dataless and self.shape != data.shape:
+        # Determine whether the class already has a defined shape,
+        # as this method is called from __init__.
+        has_shape = self.shape is not None
+        if has_shape and not dataless and self.shape != data.shape:
             # The _ONLY_ data reshape permitted is converting a 0-dimensional
             # array i.e. self.shape == () into a 1-dimensional array of length
             # one i.e. data.shape == (1,)
-            if (not is_lazy_data(data)) and dataless:
-                self._shape = self.shape
-            elif self.shape or data.shape != (1,):
+            if self.shape or data.shape != (1,):
                 emsg = "Require data with shape {!r}, got {!r}."
                 raise ValueError(emsg.format(self.shape, data.shape))
 
diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index c8201726ba..9308f4fa35 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -454,6 +454,48 @@ def test_nd_lazy_to_nd_lazy(self):
         self.assertTrue(dm.has_lazy_data())
         self.assertArrayEqual(dm.data, lazy_array.compute())
 
+    def test_nd_lazy_to_dataless(self):
+        shape = (2, 3, 4)
+        size = np.prod(shape)
+        real_array = np.arange(size).reshape(shape)
+        lazy_array = as_lazy_data(real_array)
+        dm = DataManager(lazy_array * 10)
+        self.assertTrue(dm.has_lazy_data())
+        dm.data = None
+        self.assertTrue(dm.core_data() is None)
+        self.assertTrue(dm.shape == shape)
+
+    def test_nd_real_to_dataless(self):
+        shape = (2, 3, 4)
+        size = np.prod(shape)
+        real_array = np.arange(size).reshape(shape)
+        dm = DataManager(real_array)
+        self.assertFalse(dm.has_lazy_data())
+        dm.data = None
+        self.assertTrue(dm.core_data() is None)
+        self.assertTrue(dm.shape == shape)
+
+    def test_dataless_to_nd_lazy(self):
+        shape = (2, 3, 4)
+        size = np.prod(shape)
+        real_array = np.arange(size).reshape(shape)
+        lazy_array = as_lazy_data(real_array)
+        dm = DataManager(None, shape)
+        self.assertTrue(dm.shape == shape)
+        dm.data = lazy_array
+        self.assertTrue(dm.has_lazy_data())
+        self.assertArrayEqual(dm.data, lazy_array.compute())
+
+    def test_dataless_to_nd_real(self):
+        shape = (2, 3, 4)
+        size = np.prod(shape)
+        real_array = np.arange(size).reshape(shape)
+        dm = DataManager(None, shape)
+        self.assertTrue(dm.data is None)
+        dm.data = real_array
+        self.assertFalse(dm.has_lazy_data())
+        self.assertArrayEqual(dm.data, real_array)
+
     def test_coerce_to_ndarray(self):
         shape = (2, 3)
         size = np.prod(shape)
@@ -527,6 +569,11 @@ def test_shape_nd(self):
         dm = DataManager(lazy_array)
         self.assertEqual(dm.shape, shape)
 
+    def test_shape_dataless(self):
+        shape = (2, 3, 4)
+        dm = DataManager(None, shape)
+        self.assertEqual(dm.shape, shape)
+
 
 class Test_copy(tests.IrisTest):
     def setUp(self):

From 95c09e134981db8d5ead713b87d54096f366efb7 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Thu, 12 Dec 2024 11:20:46 +0000
Subject: [PATCH 13/48] refactored tests

---
 .../tests/unit/data_manager/test_DataManager.py | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 9308f4fa35..472bfdff1c 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -545,10 +545,10 @@ def test_ndim_nd(self):
         shape = (2, 3, 4)
         real_array = np.arange(24).reshape(shape)
         dm = DataManager(real_array)
-        self.assertEqual(dm.ndim, len(shape))
+        self.assertEqual(dm.ndim, 3)
         lazy_array = as_lazy_data(real_array)
         dm = DataManager(lazy_array)
-        self.assertEqual(dm.ndim, len(shape))
+        self.assertEqual(dm.ndim, 3)
 
 
 class Test_shape(tests.IrisTest):
@@ -569,10 +569,21 @@ def test_shape_nd(self):
         dm = DataManager(lazy_array)
         self.assertEqual(dm.shape, shape)
 
-    def test_shape_dataless(self):
+    def test_shape_data_to_dataless(self):
         shape = (2, 3, 4)
+        real_array = np.arange(24).reshape(shape)
         dm = DataManager(None, shape)
         self.assertEqual(dm.shape, shape)
+        dm.data = real_array
+        self.assertEqual(dm.shape, shape)
+
+    def test_shape_dataless_to_data(self):
+        shape = (2, 3, 4)
+        real_array = np.arange(24).reshape(shape)
+        dm = DataManager(real_array)
+        self.assertEqual(dm.shape, shape)
+        dm.data = None
+        self.assertEqual(dm.shape, shape)
 
 
 class Test_copy(tests.IrisTest):

From b2fb2f8de32d48665cfbc4fea45909994293f91f Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Thu, 12 Dec 2024 11:31:16 +0000
Subject: [PATCH 14/48] removed shape asserts within data tests

---
 lib/iris/tests/unit/data_manager/test_DataManager.py | 2 --
 1 file changed, 2 deletions(-)

diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 472bfdff1c..c95b7a0dfa 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -463,7 +463,6 @@ def test_nd_lazy_to_dataless(self):
         self.assertTrue(dm.has_lazy_data())
         dm.data = None
         self.assertTrue(dm.core_data() is None)
-        self.assertTrue(dm.shape == shape)
 
     def test_nd_real_to_dataless(self):
         shape = (2, 3, 4)
@@ -473,7 +472,6 @@ def test_nd_real_to_dataless(self):
         self.assertFalse(dm.has_lazy_data())
         dm.data = None
         self.assertTrue(dm.core_data() is None)
-        self.assertTrue(dm.shape == shape)
 
     def test_dataless_to_nd_lazy(self):
         shape = (2, 3, 4)

From c6510a56c064f0296a22ab4d49327b56d1f44df8 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Thu, 12 Dec 2024 15:35:35 +0000
Subject: [PATCH 15/48] copy cube now has FUTURE behaviour

---
 lib/iris/__init__.py      |  9 ++++++---
 lib/iris/_data_manager.py | 12 ++++++++----
 lib/iris/cube.py          |  1 +
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/lib/iris/__init__.py b/lib/iris/__init__.py
index d4454efe89..98b3275769 100644
--- a/lib/iris/__init__.py
+++ b/lib/iris/__init__.py
@@ -143,7 +143,7 @@ def callback(cube, field, filename):
 class Future(threading.local):
     """Run-time configuration controller."""
 
-    def __init__(self, datum_support=False, pandas_ndim=False, save_split_attrs=False):
+    def __init__(self, datum_support=False, pandas_ndim=False, save_split_attrs=False, dataless_cube=False):
         """Container for run-time options controls.
 
         To adjust the values simply update the relevant attribute from
@@ -181,6 +181,7 @@ def __init__(self, datum_support=False, pandas_ndim=False, save_split_attrs=Fals
         self.__dict__["datum_support"] = datum_support
         self.__dict__["pandas_ndim"] = pandas_ndim
         self.__dict__["save_split_attrs"] = save_split_attrs
+        self.__dict__["dataless_cube"] = dataless_cube
 
         # TODO: next major release: set IrisDeprecation to subclass
         #  DeprecationWarning instead of UserWarning.
@@ -188,8 +189,8 @@ def __init__(self, datum_support=False, pandas_ndim=False, save_split_attrs=Fals
     def __repr__(self):
         # msg = ('Future(example_future_flag={})')
         # return msg.format(self.example_future_flag)
-        msg = "Future(datum_support={}, pandas_ndim={}, save_split_attrs={})"
-        return msg.format(self.datum_support, self.pandas_ndim, self.save_split_attrs)
+        msg = "Future(datum_support={}, pandas_ndim={}, save_split_attrs={}, dataless_cubes={})"
+        return msg.format(self.datum_support, self.pandas_ndim, self.save_split_attrs, self.dataless_cube)
 
     # deprecated_options = {'example_future_flag': 'warning',}
     deprecated_options: dict[str, Literal["error", "warning"]] = {}
@@ -832,3 +833,5 @@ def use_plugin(plugin_name):
     significance of the import statement and warn that it is an unused import.
     """
     importlib.import_module(f"iris.plugins.{plugin_name}")
+
+MAINTAIN_DATA = "MAINTAINDATA"
\ No newline at end of file
diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 35677ddac6..e0093a746f 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -157,9 +157,14 @@ def _deepcopy(self, memo, data=None):
         :class:`~iris._data_manager.DataManager` instance.
 
         """
-        # @TODO how to ask copy to make an empty cube, special value? flag?
+        shape = None
         try:
-            if data is None:
+            if (iris.FUTURE.dataless_cube and data is None):
+                shape = self.shape
+            elif (
+                    (iris.FUTURE.dataless_cube and data == iris.MAINTAIN_DATA)
+                    or (data is None)
+                ):
                 # Copy the managed data.
                 if self.has_lazy_data():
                     data = copy.deepcopy(self._lazy_array, memo)
@@ -172,11 +177,10 @@ def _deepcopy(self, memo, data=None):
                 dm_check.data = data
                 # If the replacement data is valid, then use it but
                 # without copying it.
-            result = DataManager(data)
+            result = DataManager(data=data, shape=shape)
         except ValueError as error:
             emsg = "Cannot copy {!r} - {}"
             raise ValueError(emsg.format(type(self).__name__, error))
-
         return result
 
     @property
diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index 074b42197b..e90cac7b45 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -4084,6 +4084,7 @@ def _deepcopy(self, memo, data=None):
             aux_coords_and_dims=new_aux_coords_and_dims,
             cell_measures_and_dims=new_cell_measures_and_dims,
             ancillary_variables_and_dims=new_ancillary_variables_and_dims,
+            shape=(dm.shape if dm.core_data() is None else None)
         )
 
         new_cube.metadata = deepcopy(self.metadata, memo)

From af7d727d9635063169a045d8863b5986fe5acb90 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Thu, 12 Dec 2024 15:38:26 +0000
Subject: [PATCH 16/48] pre-commit

---
 lib/iris/__init__.py      | 18 +++++++++++++++---
 lib/iris/_data_manager.py |  9 ++++-----
 lib/iris/cube.py          |  2 +-
 3 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/lib/iris/__init__.py b/lib/iris/__init__.py
index 98b3275769..37b6a89c9a 100644
--- a/lib/iris/__init__.py
+++ b/lib/iris/__init__.py
@@ -143,7 +143,13 @@ def callback(cube, field, filename):
 class Future(threading.local):
     """Run-time configuration controller."""
 
-    def __init__(self, datum_support=False, pandas_ndim=False, save_split_attrs=False, dataless_cube=False):
+    def __init__(
+        self,
+        datum_support=False,
+        pandas_ndim=False,
+        save_split_attrs=False,
+        dataless_cube=False,
+    ):
         """Container for run-time options controls.
 
         To adjust the values simply update the relevant attribute from
@@ -190,7 +196,12 @@ def __repr__(self):
         # msg = ('Future(example_future_flag={})')
         # return msg.format(self.example_future_flag)
         msg = "Future(datum_support={}, pandas_ndim={}, save_split_attrs={}, dataless_cubes={})"
-        return msg.format(self.datum_support, self.pandas_ndim, self.save_split_attrs, self.dataless_cube)
+        return msg.format(
+            self.datum_support,
+            self.pandas_ndim,
+            self.save_split_attrs,
+            self.dataless_cube,
+        )
 
     # deprecated_options = {'example_future_flag': 'warning',}
     deprecated_options: dict[str, Literal["error", "warning"]] = {}
@@ -834,4 +845,5 @@ def use_plugin(plugin_name):
     """
     importlib.import_module(f"iris.plugins.{plugin_name}")
 
-MAINTAIN_DATA = "MAINTAINDATA"
\ No newline at end of file
+
+MAINTAIN_DATA = "MAINTAINDATA"
diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index e0093a746f..5f30d33cec 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -159,12 +159,11 @@ def _deepcopy(self, memo, data=None):
         """
         shape = None
         try:
-            if (iris.FUTURE.dataless_cube and data is None):
+            if iris.FUTURE.dataless_cube and data is None:
                 shape = self.shape
-            elif (
-                    (iris.FUTURE.dataless_cube and data == iris.MAINTAIN_DATA)
-                    or (data is None)
-                ):
+            elif (iris.FUTURE.dataless_cube and data == iris.MAINTAIN_DATA) or (
+                data is None
+            ):
                 # Copy the managed data.
                 if self.has_lazy_data():
                     data = copy.deepcopy(self._lazy_array, memo)
diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index e90cac7b45..902999b112 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -4084,7 +4084,7 @@ def _deepcopy(self, memo, data=None):
             aux_coords_and_dims=new_aux_coords_and_dims,
             cell_measures_and_dims=new_cell_measures_and_dims,
             ancillary_variables_and_dims=new_ancillary_variables_and_dims,
-            shape=(dm.shape if dm.core_data() is None else None)
+            shape=(dm.shape if dm.core_data() is None else None),
         )
 
         new_cube.metadata = deepcopy(self.metadata, memo)

From 144e164b078b2c0ad77bfb80e439e3279b9abeb7 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Mon, 16 Dec 2024 11:46:02 +0000
Subject: [PATCH 17/48] written tests for cube.copy

---
 lib/iris/tests/unit/cube/test_Cube.py | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index 1f01efd90f..4104b74ffc 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -2082,6 +2082,21 @@ def test_copy_cell_measures(self):
         cube.add_cell_measure(cms, 0)
         self._check_copy(cube, cube.copy())
 
+    def test_copy_new_data(self):
+        cube = stock.simple_3d()
+        new_data = np.ones(cube.shape)
+        new_cube = cube.copy(data=new_data)
+        assert new_cube.metadata == new_cube.metadata
+        _shared_utils.assert_array_equal(new_cube.data, new_data)
+
+    def test_copy_remove_data(self):
+        iris.FUTURE.dataless_cube = True
+        cube = stock.simple_3d()
+        new_cube = cube.copy()
+        assert new_cube.metadata == cube.metadata
+        assert new_cube.data is None
+        assert new_cube.shape == cube.shape
+
     def test__masked_emptymask(self):
         cube = Cube(ma.array([0, 1]))
         self._check_copy(cube, cube.copy())

From ba84553bd9b39b8da73875c218a2aed9c4c6bb63 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Mon, 16 Dec 2024 13:20:19 +0000
Subject: [PATCH 18/48] fixed cbe.copy failure in dim coords

---
 lib/iris/coords.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/iris/coords.py b/lib/iris/coords.py
index 06a271cbba..bdd2ac231e 100644
--- a/lib/iris/coords.py
+++ b/lib/iris/coords.py
@@ -2708,7 +2708,8 @@ def __deepcopy__(self, memo):  # numpydoc ignore=SS02
         """
         new_coord = copy.deepcopy(super(), memo)
         # Ensure points and bounds arrays are read-only.
-        new_coord._values_dm.data.flags.writeable = False
+        if not (new_coord._values_dm.data is None and iris.FUTURE.dataless_cube):
+            new_coord._values_dm.data.flags.writeable = False
         if new_coord._bounds_dm is not None:
             new_coord._bounds_dm.data.flags.writeable = False
         return new_coord

From ea3c1505c5c75adf834d3b61ddf48268584b49ca Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Mon, 16 Dec 2024 13:24:57 +0000
Subject: [PATCH 19/48] experimenting with 4d cube with everything; doesn't run
 locally

---
 lib/iris/tests/unit/cube/test_Cube.py | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index 4104b74ffc..b6332af97d 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -642,8 +642,8 @@ def _assert_warn_collapse_without_weight(self, coords, warn):
         msg = "Collapsing spatial coordinate {!r} without weighting"
         for coord in coords:
             assert (
-                mock.call(msg.format(coord), category=IrisUserWarning)
-                in warn.call_args_list
+                    mock.call(msg.format(coord), category=IrisUserWarning)
+                    in warn.call_args_list
             )
 
     def _assert_nowarn_collapse_without_weight(self, coords, warn):
@@ -735,7 +735,7 @@ def _assert_warn_cannot_check_contiguity(self, warn):
                 f"'{coord}'. Ignoring bounds."
             )
             assert (
-                mock.call(msg, category=IrisVagueMetadataWarning) in warn.call_args_list
+                    mock.call(msg, category=IrisVagueMetadataWarning) in warn.call_args_list
             )
 
     def _assert_cube_as_expected(self, cube):
@@ -839,7 +839,7 @@ def test_long_components(self):
                     # For lines with any columns : check that columns are where expected
                     for col_ind in colon_inds:
                         # Chop out chars before+after each expected column.
-                        assert line[col_ind - 1 : col_ind + 2] == " x "
+                        assert line[col_ind - 1: col_ind + 2] == " x "
 
             # Finally also: compare old with new, but replacing new name and ignoring spacing differences
             def collapse_space(string):
@@ -2091,7 +2091,7 @@ def test_copy_new_data(self):
 
     def test_copy_remove_data(self):
         iris.FUTURE.dataless_cube = True
-        cube = stock.simple_3d()
+        cube = stock.realistic_4d_w_everything()
         new_cube = cube.copy()
         assert new_cube.metadata == cube.metadata
         assert new_cube.data is None

From 0cec4caf1a9061621774b200e9ff5f431b5c7894 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Mon, 16 Dec 2024 13:26:14 +0000
Subject: [PATCH 20/48] pre-c

---
 lib/iris/tests/unit/cube/test_Cube.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index b6332af97d..5595bc29c9 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -642,8 +642,8 @@ def _assert_warn_collapse_without_weight(self, coords, warn):
         msg = "Collapsing spatial coordinate {!r} without weighting"
         for coord in coords:
             assert (
-                    mock.call(msg.format(coord), category=IrisUserWarning)
-                    in warn.call_args_list
+                mock.call(msg.format(coord), category=IrisUserWarning)
+                in warn.call_args_list
             )
 
     def _assert_nowarn_collapse_without_weight(self, coords, warn):
@@ -735,7 +735,7 @@ def _assert_warn_cannot_check_contiguity(self, warn):
                 f"'{coord}'. Ignoring bounds."
             )
             assert (
-                    mock.call(msg, category=IrisVagueMetadataWarning) in warn.call_args_list
+                mock.call(msg, category=IrisVagueMetadataWarning) in warn.call_args_list
             )
 
     def _assert_cube_as_expected(self, cube):
@@ -839,7 +839,7 @@ def test_long_components(self):
                     # For lines with any columns : check that columns are where expected
                     for col_ind in colon_inds:
                         # Chop out chars before+after each expected column.
-                        assert line[col_ind - 1: col_ind + 2] == " x "
+                        assert line[col_ind - 1 : col_ind + 2] == " x "
 
             # Finally also: compare old with new, but replacing new name and ignoring spacing differences
             def collapse_space(string):

From 6ed270de6cf746edb75df173fecb2ccb79ac6c4d Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Mon, 16 Dec 2024 13:57:07 +0000
Subject: [PATCH 21/48] edited Coord.copy

---
 lib/iris/coords.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/lib/iris/coords.py b/lib/iris/coords.py
index bdd2ac231e..7888f44e4f 100644
--- a/lib/iris/coords.py
+++ b/lib/iris/coords.py
@@ -200,7 +200,7 @@ def _sanitise_array(self, src, ndmin):
             else:
                 extended_shape = tuple([1] * ndims_missing + list(src.shape))
                 result = src.reshape(extended_shape)
-        else:
+        elif src is None:
             # Real data : a few more things to do in this case.
             # Ensure the array is writeable.
             # NB. Returns the *same object* if src is already writeable.
@@ -1535,6 +1535,10 @@ def copy(self, points=None, bounds=None):
         """
         if points is None and bounds is not None:
             raise ValueError("If bounds are specified, points must also be specified")
+        if points is None and iris.FUTURE.dataless_cube:
+            # dataless coords are not currently implemented, so points should never
+            # be removed in a copy
+            points = iris.MAINTAIN_DATA
 
         new_coord = super().copy(values=points)
         if points is not None:

From d79ad270810c382f68c80005e75cf8de4fadfa3c Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Mon, 16 Dec 2024 14:05:14 +0000
Subject: [PATCH 22/48] Revert "edited Coord.copy"

This reverts commit 6ed270de6cf746edb75df173fecb2ccb79ac6c4d.
---
 lib/iris/coords.py | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/lib/iris/coords.py b/lib/iris/coords.py
index 7888f44e4f..bdd2ac231e 100644
--- a/lib/iris/coords.py
+++ b/lib/iris/coords.py
@@ -200,7 +200,7 @@ def _sanitise_array(self, src, ndmin):
             else:
                 extended_shape = tuple([1] * ndims_missing + list(src.shape))
                 result = src.reshape(extended_shape)
-        elif src is None:
+        else:
             # Real data : a few more things to do in this case.
             # Ensure the array is writeable.
             # NB. Returns the *same object* if src is already writeable.
@@ -1535,10 +1535,6 @@ def copy(self, points=None, bounds=None):
         """
         if points is None and bounds is not None:
             raise ValueError("If bounds are specified, points must also be specified")
-        if points is None and iris.FUTURE.dataless_cube:
-            # dataless coords are not currently implemented, so points should never
-            # be removed in a copy
-            points = iris.MAINTAIN_DATA
 
         new_coord = super().copy(values=points)
         if points is not None:

From 2bb6e61a3938cfed76779d9462b4494fa2385c0b Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Mon, 16 Dec 2024 16:09:32 +0000
Subject: [PATCH 23/48] tried tearingdown FUTUREFLAG

---
 lib/iris/_data_manager.py             |  2 +-
 lib/iris/tests/unit/cube/test_Cube.py | 12 ++++++------
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 5f30d33cec..c5b7df8e1c 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -161,7 +161,7 @@ def _deepcopy(self, memo, data=None):
         try:
             if iris.FUTURE.dataless_cube and data is None:
                 shape = self.shape
-            elif (iris.FUTURE.dataless_cube and data == iris.MAINTAIN_DATA) or (
+            elif (iris.FUTURE.dataless_cube and type(data) is str and data == iris.MAINTAIN_DATA) or (
                 data is None
             ):
                 # Copy the managed data.
diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index 5595bc29c9..7dc3068866 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -2090,12 +2090,12 @@ def test_copy_new_data(self):
         _shared_utils.assert_array_equal(new_cube.data, new_data)
 
     def test_copy_remove_data(self):
-        iris.FUTURE.dataless_cube = True
-        cube = stock.realistic_4d_w_everything()
-        new_cube = cube.copy()
-        assert new_cube.metadata == cube.metadata
-        assert new_cube.data is None
-        assert new_cube.shape == cube.shape
+        with iris.FUTURE.context(dataless_cube = True):
+            cube = stock.simple_3d()
+            new_cube = cube.copy()
+            assert new_cube.metadata == cube.metadata
+            assert new_cube.data is None
+            assert new_cube.shape == cube.shape
 
     def test__masked_emptymask(self):
         cube = Cube(ma.array([0, 1]))

From 063e45b8e3dd7583f44d4bc44f0ccde336e892ae Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Mon, 16 Dec 2024 16:11:48 +0000
Subject: [PATCH 24/48] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 lib/iris/_data_manager.py             | 8 +++++---
 lib/iris/tests/unit/cube/test_Cube.py | 2 +-
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index c5b7df8e1c..d27d759161 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -161,9 +161,11 @@ def _deepcopy(self, memo, data=None):
         try:
             if iris.FUTURE.dataless_cube and data is None:
                 shape = self.shape
-            elif (iris.FUTURE.dataless_cube and type(data) is str and data == iris.MAINTAIN_DATA) or (
-                data is None
-            ):
+            elif (
+                iris.FUTURE.dataless_cube
+                and type(data) is str
+                and data == iris.MAINTAIN_DATA
+            ) or (data is None):
                 # Copy the managed data.
                 if self.has_lazy_data():
                     data = copy.deepcopy(self._lazy_array, memo)
diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index 7dc3068866..cf832719a0 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -2090,7 +2090,7 @@ def test_copy_new_data(self):
         _shared_utils.assert_array_equal(new_cube.data, new_data)
 
     def test_copy_remove_data(self):
-        with iris.FUTURE.context(dataless_cube = True):
+        with iris.FUTURE.context(dataless_cube=True):
             cube = stock.simple_3d()
             new_cube = cube.copy()
             assert new_cube.metadata == cube.metadata

From 79d7d6720b3eb63b320074dcced33480ba02add9 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Tue, 17 Dec 2024 10:22:20 +0000
Subject: [PATCH 25/48] made dataless copy opt-in behaviour

---
 lib/iris/__init__.py                  |  7 ++-----
 lib/iris/_data_manager.py             | 11 ++++-------
 lib/iris/coords.py                    |  3 +--
 lib/iris/tests/unit/cube/test_Cube.py | 11 +++++------
 4 files changed, 12 insertions(+), 20 deletions(-)

diff --git a/lib/iris/__init__.py b/lib/iris/__init__.py
index 37b6a89c9a..5a6920d4ce 100644
--- a/lib/iris/__init__.py
+++ b/lib/iris/__init__.py
@@ -148,7 +148,6 @@ def __init__(
         datum_support=False,
         pandas_ndim=False,
         save_split_attrs=False,
-        dataless_cube=False,
     ):
         """Container for run-time options controls.
 
@@ -187,7 +186,6 @@ def __init__(
         self.__dict__["datum_support"] = datum_support
         self.__dict__["pandas_ndim"] = pandas_ndim
         self.__dict__["save_split_attrs"] = save_split_attrs
-        self.__dict__["dataless_cube"] = dataless_cube
 
         # TODO: next major release: set IrisDeprecation to subclass
         #  DeprecationWarning instead of UserWarning.
@@ -195,12 +193,11 @@ def __init__(
     def __repr__(self):
         # msg = ('Future(example_future_flag={})')
         # return msg.format(self.example_future_flag)
-        msg = "Future(datum_support={}, pandas_ndim={}, save_split_attrs={}, dataless_cubes={})"
+        msg = "Future(datum_support={}, pandas_ndim={}, save_split_attrs={})"
         return msg.format(
             self.datum_support,
             self.pandas_ndim,
             self.save_split_attrs,
-            self.dataless_cube,
         )
 
     # deprecated_options = {'example_future_flag': 'warning',}
@@ -846,4 +843,4 @@ def use_plugin(plugin_name):
     importlib.import_module(f"iris.plugins.{plugin_name}")
 
 
-MAINTAIN_DATA = "MAINTAINDATA"
+DATALESS_COPY = "NONE"
diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index d27d759161..00db3ff329 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -159,18 +159,15 @@ def _deepcopy(self, memo, data=None):
         """
         shape = None
         try:
-            if iris.FUTURE.dataless_cube and data is None:
-                shape = self.shape
-            elif (
-                iris.FUTURE.dataless_cube
-                and type(data) is str
-                and data == iris.MAINTAIN_DATA
-            ) or (data is None):
+            if data is None:
                 # Copy the managed data.
                 if self.has_lazy_data():
                     data = copy.deepcopy(self._lazy_array, memo)
                 else:
                     data = self._real_array.copy()
+            elif type(data) is str and data == iris.DATALESS_COPY:
+                shape = self.shape
+                data = None
             else:
                 # Check that the replacement data is valid relative to
                 # the currently managed data.
diff --git a/lib/iris/coords.py b/lib/iris/coords.py
index bdd2ac231e..06a271cbba 100644
--- a/lib/iris/coords.py
+++ b/lib/iris/coords.py
@@ -2708,8 +2708,7 @@ def __deepcopy__(self, memo):  # numpydoc ignore=SS02
         """
         new_coord = copy.deepcopy(super(), memo)
         # Ensure points and bounds arrays are read-only.
-        if not (new_coord._values_dm.data is None and iris.FUTURE.dataless_cube):
-            new_coord._values_dm.data.flags.writeable = False
+        new_coord._values_dm.data.flags.writeable = False
         if new_coord._bounds_dm is not None:
             new_coord._bounds_dm.data.flags.writeable = False
         return new_coord
diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index cf832719a0..54fb49ea73 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -2090,12 +2090,11 @@ def test_copy_new_data(self):
         _shared_utils.assert_array_equal(new_cube.data, new_data)
 
     def test_copy_remove_data(self):
-        with iris.FUTURE.context(dataless_cube=True):
-            cube = stock.simple_3d()
-            new_cube = cube.copy()
-            assert new_cube.metadata == cube.metadata
-            assert new_cube.data is None
-            assert new_cube.shape == cube.shape
+        cube = stock.simple_3d()
+        new_cube = cube.copy(iris.DATALESS_COPY)
+        assert new_cube.metadata == cube.metadata
+        assert new_cube.data is None
+        assert new_cube.shape == cube.shape
 
     def test__masked_emptymask(self):
         cube = Cube(ma.array([0, 1]))

From 9b52067a1ff90454327f4524ac0402c0cc895c68 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Tue, 17 Dec 2024 11:39:14 +0000
Subject: [PATCH 26/48] cube data is optional

---
 lib/iris/cube.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index 902999b112..fe497ef228 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -1190,7 +1190,7 @@ def _walk_nodes(node):
 
     def __init__(
         self,
-        data: np.typing.ArrayLike,
+        data: np.typing.ArrayLike | None = None,
         standard_name: str | None = None,
         long_name: str | None = None,
         var_name: str | None = None,
@@ -1251,6 +1251,9 @@ def __init__(
             A list of CellMeasures with dimension mappings.
         ancillary_variables_and_dims :
             A list of AncillaryVariables with dimension mappings.
+        shape :
+            An alternative to providing data, this defines the shape of the
+            cube, but initialises the cube as dataless.
 
         Examples
         --------

From 2f2b56ad8638d828c6cc31d8b3dbebae47b93f15 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Tue, 17 Dec 2024 14:38:23 +0000
Subject: [PATCH 27/48] added is_dataless, and corrected data manager
 exceptions to not be cube specific

---
 lib/iris/_data_manager.py                     | 19 ++++++++++++++-----
 lib/iris/cube.py                              | 10 ++++++++++
 .../unit/data_manager/test_DataManager.py     | 13 +++++++++++++
 3 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 00db3ff329..d7956b9f18 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -25,12 +25,18 @@ def __init__(self, data, shape=None):
         data :
             The :class:`~numpy.ndarray` or :class:`~numpy.ma.core.MaskedArray`
             real data, or :class:`~dask.array.core.Array` lazy data to be
-            managed.
+            managed. If a value of None is given, the data manager will be
+            considered dataless.
+
+        shape :
+            A tuple, representing the shape of the data manager. This can only
+            be used in the case of `data=None`, and will render the data manager
+            dataless.
 
         """
         if (shape is not None) and (data is not None):
-            msg = "A cube may not be created with both data and a custom shape."
-            raise iris.exceptions.InvalidCubeError(msg)
+            msg = "`shape` should only be provided if `data is None`"
+            raise ValueError(msg)
 
         # Initialise the instance.
         self._lazy_array = None
@@ -134,12 +140,12 @@ def _assert_axioms(self):
         overfilled = self._lazy_array is not None and self._real_array is not None
         if overfilled:
             msg = "Unexpected data state, got both lazy and real data."
-            raise iris.exceptions.InvalidCubeError(msg)
+            raise ValueError(msg)
         elif (
             empty and self._shape is None
         ):  # if I remove the second check, allows empty arrays, like old behaviour
             msg = "Unexpected data state, got no lazy or real data, and no shape."
-            raise iris.exceptions.InvalidCubeError(msg)
+            raise ValueError(msg)
 
     def _deepcopy(self, memo, data=None):
         """Perform a deepcopy of the :class:`~iris._data_manager.DataManager` instance.
@@ -290,6 +296,9 @@ def shape(self):
             result = self.core_data().shape
         return result
 
+    def is_dataless(self):
+        return (self.core_data() is None) and (self.shape is not None)
+
     def copy(self, data=None):
         """Return a deep copy of this :class:`~iris._data_manager.DataManager` instance.
 
diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index fe497ef228..fca5539135 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -2887,6 +2887,16 @@ def has_lazy_data(self) -> bool:
         """
         return self._data_manager.has_lazy_data()
 
+    def is_dataless(self) -> bool:
+        """Detail whether this :class:`~iris.cube.Cube` is dataless.
+
+        Returns
+        -------
+        bool
+
+        """
+        return self._data_manager.is_dataless()
+
     @property
     def dim_coords(self) -> tuple[DimCoord, ...]:
         """Return a tuple of all the dimension coordinates, ordered by dimension.
diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index c95b7a0dfa..02cefc1e7e 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -647,6 +647,19 @@ def test_with_lazy_array(self):
         self.assertTrue(dm.has_lazy_data())
         self.assertIs(result, dm._lazy_array)
 
+class Test_is_dataless(tests.IrisTest):
+    def setUp(self):
+        self.data = np.array(0)
+        self.shape = (0)
+
+    def test_with_data(self):
+        dm = DataManager(self.data)
+        self.assertFalse(dm.is_dataless())
+
+    def test_without_data(self):
+        dm = DataManager(None, self.shape)
+        self.assertTrue(dm.is_dataless())
+
 
 if __name__ == "__main__":
     tests.main()

From c8d2c073ae28333618a12afea066d307ac8368b1 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Tue, 17 Dec 2024 14:40:07 +0000
Subject: [PATCH 28/48] pre-c

---
 lib/iris/tests/unit/data_manager/test_DataManager.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 02cefc1e7e..e88536f975 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -647,10 +647,11 @@ def test_with_lazy_array(self):
         self.assertTrue(dm.has_lazy_data())
         self.assertIs(result, dm._lazy_array)
 
+
 class Test_is_dataless(tests.IrisTest):
     def setUp(self):
         self.data = np.array(0)
-        self.shape = (0)
+        self.shape = 0
 
     def test_with_data(self):
         dm = DataManager(self.data)

From 67fb787f8e2900c52d4369cf82bc5b6be8aa402d Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Tue, 17 Dec 2024 15:07:48 +0000
Subject: [PATCH 29/48] fixed failing tests

---
 lib/iris/tests/unit/data_manager/test_DataManager.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index e88536f975..83ba548653 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -4,8 +4,6 @@
 # See LICENSE in the root of the repository for full licensing details.
 """Unit tests for the :class:`iris._data_manager.DataManager`."""
 
-import iris.exceptions
-
 # Import iris.tests first so that some things can be initialised before
 # importing anything else.
 import iris.tests as tests  # isort:skip
@@ -170,13 +168,13 @@ def setUp(self):
     def test_array_none(self):
         self.dm._real_array = None
         emsg = "Unexpected data state, got no lazy or real data, and no shape."
-        with self.assertRaisesRegex(iris.exceptions.InvalidCubeError, emsg):
+        with self.assertRaisesRegex(ValueError, emsg):
             self.dm._assert_axioms()
 
     def test_array_all(self):
         self.dm._lazy_array = self.lazy_array
         emsg = "Unexpected data state, got both lazy and real data."
-        with self.assertRaisesRegex(iris.exceptions.InvalidCubeError, emsg):
+        with self.assertRaisesRegex(ValueError, emsg):
             self.dm._assert_axioms()
 
 

From a04245171e4ce9058628ba637c5890d706bfdaa7 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Tue, 17 Dec 2024 16:17:21 +0000
Subject: [PATCH 30/48] fixed copying from dataless

---
 lib/iris/_data_manager.py             |  4 +++-
 lib/iris/tests/unit/cube/test_Cube.py | 12 ++++++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index d7956b9f18..0382afa373 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -169,8 +169,10 @@ def _deepcopy(self, memo, data=None):
                 # Copy the managed data.
                 if self.has_lazy_data():
                     data = copy.deepcopy(self._lazy_array, memo)
-                else:
+                elif self._real_array is not None:
                     data = self._real_array.copy()
+                else:
+                    shape = self.shape
             elif type(data) is str and data == iris.DATALESS_COPY:
                 shape = self.shape
                 data = None
diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index 54fb49ea73..3d12fcc6cc 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -2082,20 +2082,28 @@ def test_copy_cell_measures(self):
         cube.add_cell_measure(cms, 0)
         self._check_copy(cube, cube.copy())
 
-    def test_copy_new_data(self):
+    def test_copy_replace_data(self):
         cube = stock.simple_3d()
         new_data = np.ones(cube.shape)
         new_cube = cube.copy(data=new_data)
         assert new_cube.metadata == new_cube.metadata
         _shared_utils.assert_array_equal(new_cube.data, new_data)
 
-    def test_copy_remove_data(self):
+    def test_copy_to_dataless(self):
         cube = stock.simple_3d()
         new_cube = cube.copy(iris.DATALESS_COPY)
         assert new_cube.metadata == cube.metadata
         assert new_cube.data is None
         assert new_cube.shape == cube.shape
 
+    def test_copy_from_dataless(self):
+        cube = stock.simple_3d()
+        cube.data = None
+        new_cube = cube.copy()
+        assert new_cube.metadata == cube.metadata
+        assert new_cube.data is cube.data
+        assert new_cube.shape == cube.shape
+
     def test__masked_emptymask(self):
         cube = Cube(ma.array([0, 1]))
         self._check_copy(cube, cube.copy())

From 21489d572fdb500fae5852a839a09afc7f89890f Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 18 Dec 2024 12:49:30 +0000
Subject: [PATCH 31/48] added in exceptions, not tested

---
 lib/iris/cube.py       | 24 +++++++++++++++++++++++-
 lib/iris/exceptions.py | 11 +++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index fca5539135..e5d0573b0d 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -174,6 +174,8 @@ def _assert_is_cube(obj):
         if not hasattr(obj, "add_aux_coord"):
             msg = r"Object {obj} cannot be put in a cubelist, as it is not a Cube."
             raise ValueError(msg)
+        elif obj.is_dataless():
+            raise iris.exceptions.DatalessError("CubeList")
 
     def _repr_html_(self):
         from iris.experimental.representation import CubeListRepresentation
@@ -1479,6 +1481,8 @@ def convert_units(self, unit: str | Unit) -> None:
 
         """
         # If the cube has units convert the data.
+        if self.is_dataless():
+            raise iris.exceptions.DatalessError("convert_units")
         if self.units.is_unknown():
             raise iris.exceptions.UnitConversionError(
                 "Cannot convert from unknown units. "
@@ -3105,6 +3109,8 @@ def subset(self, coord: AuxCoord | DimCoord) -> Cube | None:
         whole cube is returned. As such, the operation is not strict.
 
         """
+        if self.is_dataless():
+            raise iris.exceptions.DatalessError("subset")
         if not isinstance(coord, iris.coords.Coord):
             raise ValueError("coord_to_extract must be a valid Coord.")
 
@@ -3226,6 +3232,8 @@ def intersection(self, *args, **kwargs) -> Cube:
             which intersects with the requested coordinate intervals.
 
         """
+        if self.is_dataless():
+            raise iris.exceptions.DatalessError("intersection")
         result = self
         ignore_bounds = kwargs.pop("ignore_bounds", False)
         threshold = kwargs.pop("threshold", 0)
@@ -3750,6 +3758,9 @@ def slices(
             dimension index.
 
         """  # noqa: D214, D406, D407, D410, D411
+        if self.is_dataless():
+            raise iris.exceptions.DatalessError("slices")
+
         if not isinstance(ordered, bool):
             raise TypeError("'ordered' argument to slices must be boolean.")
 
@@ -3837,7 +3848,8 @@ def transpose(self, new_order: list[int] | None = None) -> None:
 
         # Transpose the data payload.
         dm = self._data_manager
-        data = dm.core_data().transpose(new_order)
+        if not self.is_dataless():
+            data = dm.core_data().transpose(new_order)
         self._data_manager = DataManager(data)
 
         dim_mapping = {src: dest for dest, src in enumerate(new_order)}
@@ -4325,6 +4337,8 @@ def collapsed(
                 cube.collapsed(['latitude', 'longitude'],
                                iris.analysis.VARIANCE)
         """
+        if self.is_dataless():
+            raise iris.exceptions.DatalessError("collapsed")
         # Update weights kwargs (if necessary) to handle different types of
         # weights
         weights_info = None
@@ -4545,6 +4559,8 @@ def aggregated_by(
                     STASH                       m01s00i024
 
         """
+        if self.is_dataless():
+            raise iris.exceptions.DatalessError("aggregated_by")
         # Update weights kwargs (if necessary) to handle different types of
         # weights
         weights_info = None
@@ -4844,6 +4860,8 @@ def rolling_window(
         """  # noqa: D214, D406, D407, D410, D411
         # Update weights kwargs (if necessary) to handle different types of
         # weights
+        if self.is_dataless():
+            raise iris.exceptions.DatalessError("rolling_window")
         weights_info = None
         if kwargs.get("weights") is not None:
             weights_info = _Weights(kwargs["weights"], self)
@@ -5049,6 +5067,8 @@ def interpolate(
             True
 
         """
+        if self.is_dataless():
+            raise iris.exceptions.DatalessError("interoplate")
         coords, points = zip(*sample_points)
         interp = scheme.interpolator(self, coords)  # type: ignore[arg-type]
         return interp(points, collapse_scalar=collapse_scalar)
@@ -5094,6 +5114,8 @@ def regrid(self, grid: Cube, scheme: iris.analysis.RegriddingScheme) -> Cube:
             this function is not applicable.
 
         """
+        if self.is_dataless():
+            raise iris.exceptions.DatalessError("regrid")
         regridder = scheme.regridder(self, grid)
         return regridder(self)
 
diff --git a/lib/iris/exceptions.py b/lib/iris/exceptions.py
index d6d2084d3c..450afce3a6 100644
--- a/lib/iris/exceptions.py
+++ b/lib/iris/exceptions.py
@@ -161,3 +161,14 @@ class CannotAddError(ValueError):
     """Raised when an object (e.g. coord) cannot be added to a :class:`~iris.cube.Cube`."""
 
     pass
+
+
+class DatalessError(ValueError):
+    """Raised when an method cannot be performed on a dataless :class:`~iris.cube.Cube`."""
+
+    def __str__(self):
+        msg = (
+            "Dataless cubes are still early in implementation, and dataless {} "
+            "operations are not currently supported."
+        )
+        return msg.format(super().__str__())

From a059c5e483429d32a133af88b8e5a76484e7b25e Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 18 Dec 2024 13:54:41 +0000
Subject: [PATCH 32/48] renamed DATALESS_COPY to DATALESS, and added comment

---
 lib/iris/__init__.py                  | 3 ++-
 lib/iris/_data_manager.py             | 2 +-
 lib/iris/tests/unit/cube/test_Cube.py | 2 +-
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/lib/iris/__init__.py b/lib/iris/__init__.py
index 5a6920d4ce..90d5ac62d2 100644
--- a/lib/iris/__init__.py
+++ b/lib/iris/__init__.py
@@ -843,4 +843,5 @@ def use_plugin(plugin_name):
     importlib.import_module(f"iris.plugins.{plugin_name}")
 
 
-DATALESS_COPY = "NONE"
+# To be used when copying a cube to make the new cube dataless.
+DATALESS = "NONE"
diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 0382afa373..ab7d1bef89 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -173,7 +173,7 @@ def _deepcopy(self, memo, data=None):
                     data = self._real_array.copy()
                 else:
                     shape = self.shape
-            elif type(data) is str and data == iris.DATALESS_COPY:
+            elif type(data) is str and data == iris.DATALESS:
                 shape = self.shape
                 data = None
             else:
diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index 3d12fcc6cc..a2bed9685b 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -2091,7 +2091,7 @@ def test_copy_replace_data(self):
 
     def test_copy_to_dataless(self):
         cube = stock.simple_3d()
-        new_cube = cube.copy(iris.DATALESS_COPY)
+        new_cube = cube.copy(iris.DATALESS)
         assert new_cube.metadata == cube.metadata
         assert new_cube.data is None
         assert new_cube.shape == cube.shape

From 8178809bfa31bfb7be83dc1cd632c139d889d528 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 18 Dec 2024 14:24:39 +0000
Subject: [PATCH 33/48] made DATALESS docstring a docstring, rather than a
 comment

---
 lib/iris/__init__.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/iris/__init__.py b/lib/iris/__init__.py
index 82c1cec41b..fd1b757ed1 100644
--- a/lib/iris/__init__.py
+++ b/lib/iris/__init__.py
@@ -853,5 +853,5 @@ def use_plugin(plugin_name):
     importlib.import_module(f"iris.plugins.{plugin_name}")
 
 
-# To be used when copying a cube to make the new cube dataless.
+#: To be used when copying a cube to make the new cube dataless.
 DATALESS = "NONE"

From 2123b8f1c274c03a0e36e615aaff4a6adc95f438 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 18 Dec 2024 14:49:22 +0000
Subject: [PATCH 34/48] added whatsnew

---
 docs/src/whatsnew/latest.rst | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/docs/src/whatsnew/latest.rst b/docs/src/whatsnew/latest.rst
index ae13b8a883..0c3bac110e 100644
--- a/docs/src/whatsnew/latest.rst
+++ b/docs/src/whatsnew/latest.rst
@@ -38,6 +38,17 @@ This document explains the changes made to Iris for this release
    your code for new floating point problems if activating this (e.g. when
    using the :class:`~iris.Constraint` API). (:pull:`6260`)
 
+#. `@ESadek-MO`_ made :attr:`~iris.cube.Cube.data` optional in a
+   :class:`~iris.cube.Cube`, when :attr:`~iris.cube.Cube.shape` is provided
+   instead. `dataless cubes` can currently be used as targets in regridding, or
+   for templates to add data to at a later time.
+
+   This is the first step in making `dataless cubes`. Currently, most cube methods
+   don't work on `dataless cubes`, and will raise in an error if attempted.
+   :meth:`~iris.cube.Cube.transpose` will work, as will :meth:`~iris.cube.Cube.copy`.
+   `my_cube.copy(data = iris.DATALESS)` will copy the cube and remove data in
+   the process.
+   (:issue:`4447`, :pull:`6253`)
 
 🐛 Bugs Fixed
 =============

From 703bbb6826c3a67d7ac8b35aaf65bd622a515015 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 18 Dec 2024 16:45:43 +0000
Subject: [PATCH 35/48] added cubelist errors

---
 lib/iris/_concatenate.py | 6 +++++-
 lib/iris/_lazy_data.py   | 6 +++++-
 lib/iris/_merge.py       | 4 ++++
 lib/iris/cube.py         | 3 ---
 4 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/lib/iris/_concatenate.py b/lib/iris/_concatenate.py
index ac9e699790..6caee79c4f 100644
--- a/lib/iris/_concatenate.py
+++ b/lib/iris/_concatenate.py
@@ -572,7 +572,11 @@ def concatenate(
         A :class:`iris.cube.CubeList` of concatenated :class:`iris.cube.Cube` instances.
 
     """
-    cube_signatures = [_CubeSignature(cube) for cube in cubes]
+    cube_signatures = []
+    for cube in cubes:
+        if cube.is_dataless():
+            raise iris.exceptions.DatalessError("concatenate")
+        cube_signatures.append(_CubeSignature(cube))
 
     proto_cubes: list[_ProtoCube] = []
     # Initialise the nominated axis (dimension) of concatenation
diff --git a/lib/iris/_lazy_data.py b/lib/iris/_lazy_data.py
index a3dfa1edb4..e3b95e685a 100644
--- a/lib/iris/_lazy_data.py
+++ b/lib/iris/_lazy_data.py
@@ -16,6 +16,7 @@
 import dask.array as da
 import dask.config
 import dask.utils
+import iris.exceptions
 import numpy as np
 import numpy.ma as ma
 
@@ -317,7 +318,10 @@ def _co_realise_lazy_arrays(arrays):
         # Note : in some cases dask (and numpy) will return a scalar
         # numpy.int/numpy.float object rather than an ndarray.
         # Recorded in https://github.com/dask/dask/issues/2111.
-        real_out = np.asanyarray(real_out)
+        if real_out is not None:
+            real_out = np.asanyarray(real_out)
+        else:
+            raise iris.exceptions.DatalessError("realising")
         if isinstance(real_out, ma.core.MaskedConstant):
             # Convert any masked constants into NumPy masked arrays.
             # NOTE: in this case, also apply the original lazy-array dtype, as
diff --git a/lib/iris/_merge.py b/lib/iris/_merge.py
index 5e00d9b2f0..633ac0f2da 100644
--- a/lib/iris/_merge.py
+++ b/lib/iris/_merge.py
@@ -1109,6 +1109,8 @@ def __init__(self, cube):
         source-cube.
 
         """
+        if cube.is_dataless():
+            raise iris.exceptions.DatalessError("merge")
         # Default hint ordering for candidate dimension coordinates.
         self._hints = [
             "time",
@@ -1289,6 +1291,8 @@ def register(self, cube, error_on_mismatch=False):
             this :class:`ProtoCube`.
 
         """
+        if cube.is_dataless():
+            raise iris.exceptions.DatalessError("merge")
         cube_signature = self._cube_signature
         other = self._build_signature(cube)
         match = cube_signature.match(other, error_on_mismatch)
diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index e5d0573b0d..744502e390 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -174,8 +174,6 @@ def _assert_is_cube(obj):
         if not hasattr(obj, "add_aux_coord"):
             msg = r"Object {obj} cannot be put in a cubelist, as it is not a Cube."
             raise ValueError(msg)
-        elif obj.is_dataless():
-            raise iris.exceptions.DatalessError("CubeList")
 
     def _repr_html_(self):
         from iris.experimental.representation import CubeListRepresentation
@@ -412,7 +410,6 @@ def merge_cube(self):
         """
         if not self:
             raise ValueError("can't merge an empty CubeList")
-
         # Register each of our cubes with a single ProtoCube.
         proto_cube = iris._merge.ProtoCube(self[0])
         for c in self[1:]:

From 33649c11260761937a147f259b7ca66365cc153f Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 18 Dec 2024 16:46:30 +0000
Subject: [PATCH 36/48] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 lib/iris/_lazy_data.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/iris/_lazy_data.py b/lib/iris/_lazy_data.py
index e3b95e685a..cfa5fde4b7 100644
--- a/lib/iris/_lazy_data.py
+++ b/lib/iris/_lazy_data.py
@@ -16,10 +16,11 @@
 import dask.array as da
 import dask.config
 import dask.utils
-import iris.exceptions
 import numpy as np
 import numpy.ma as ma
 
+import iris.exceptions
+
 
 def non_lazy(func):
     """Turn a lazy function into a function that returns a result immediately."""

From 021bd3135199c72ee2ad896aa78a0ad0bd0a0dee Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 18 Dec 2024 17:26:04 +0000
Subject: [PATCH 37/48] review comments

---
 lib/iris/__init__.py      |   7 ++-
 lib/iris/_data_manager.py | 111 +++++++++++++++++++-------------------
 2 files changed, 57 insertions(+), 61 deletions(-)

diff --git a/lib/iris/__init__.py b/lib/iris/__init__.py
index fd1b757ed1..64d1fe64c9 100644
--- a/lib/iris/__init__.py
+++ b/lib/iris/__init__.py
@@ -118,6 +118,7 @@ def callback(cube, field, filename):
 __all__ = [
     "AttributeConstraint",
     "Constraint",
+    "DATALESS",
     "FUTURE",
     "Future",
     "IrisDeprecation",
@@ -139,6 +140,8 @@ def callback(cube, field, filename):
 AttributeConstraint = iris._constraints.AttributeConstraint
 NameConstraint = iris._constraints.NameConstraint
 
+#: To be used when copying a cube to make the new cube dataless.
+DATALESS = "NONE"
 
 class Future(threading.local):
     """Run-time configuration controller."""
@@ -851,7 +854,3 @@ def use_plugin(plugin_name):
     significance of the import statement and warn that it is an unused import.
     """
     importlib.import_module(f"iris.plugins.{plugin_name}")
-
-
-#: To be used when copying a cube to make the new cube dataless.
-DATALESS = "NONE"
diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 5102c9c2f9..2268ec39f5 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -22,28 +22,29 @@ def __init__(self, data, shape=None):
 
         Parameters
         ----------
-        data :
+        data : np.typing.ArrayLike, optional
             The :class:`~numpy.ndarray` or :class:`~numpy.ma.core.MaskedArray`
             real data, or :class:`~dask.array.core.Array` lazy data to be
             managed. If a value of None is given, the data manager will be
             considered dataless.
 
-        shape :
+        shape : tuple, optional
             A tuple, representing the shape of the data manager. This can only
-            be used in the case of `data=None`, and will render the data manager
+            be used in the case of ``data=None``, and will render the data manager
             dataless.
 
         """
         if (shape is not None) and (data is not None):
-            msg = "`shape` should only be provided if `data is None`"
+            msg = '"shape" should only be provided if "data" is None'
             raise ValueError(msg)
 
+        self._shape = shape
+
         # Initialise the instance.
         self._lazy_array = None
         self._real_array = None
 
         # Assign the data payload to be managed.
-        self._shape = shape
         self.data = data
 
     def __copy__(self):
@@ -136,16 +137,16 @@ def __repr__(self):
     def _assert_axioms(self):
         """Definition of the manager state, that should never be violated."""
         # Ensure there is a valid data state.
-        empty = self._lazy_array is None and self._real_array is None
-        overfilled = self._lazy_array is not None and self._real_array is not None
-        if overfilled:
-            msg = "Unexpected data state, got both lazy and real data."
-            raise ValueError(msg)
-        elif (
-            empty and self._shape is None
-        ):  # if I remove the second check, allows empty arrays, like old behaviour
-            msg = "Unexpected data state, got no lazy or real data, and no shape."
-            raise ValueError(msg)
+        is_lazy = self._lazy_array is not None
+        is_real = self._real_array is not None
+
+        if not (is_lazy ^ is_real):
+            if is_lazy and is_real:
+                msg = "Unexpected data state, got both lazy and real data."
+                raise ValueError(msg)
+            if not self._shape and not (is_lazy or is_real):
+                msg = "Unexpected data state, got no lazy or real data, and no shape."
+                raise ValueError(msg)
 
     def _deepcopy(self, memo, data=None):
         """Perform a deepcopy of the :class:`~iris._data_manager.DataManager` instance.
@@ -172,7 +173,7 @@ def _deepcopy(self, memo, data=None):
                 elif self._real_array is not None:
                     data = self._real_array.copy()
                 else:
-                    shape = self.shape
+                    shape = self._shape
             elif type(data) is str and data == iris.DATALESS:
                 shape = self.shape
                 data = None
@@ -192,6 +193,7 @@ def _deepcopy(self, memo, data=None):
     @property
     def data(self):
         """Return the real data. Any lazy data being managed will be realised.
+        ``None`` will be returned if in the dataless state.
 
         Returns
         -------
@@ -236,47 +238,46 @@ def data(self, data):
         data :
             The :class:`~numpy.ndarray` or :class:`~numpy.ma.core.MaskedArray`
             real data, or :class:`~dask.array.core.Array` lazy data to be
-            managed.
+            managed. If data is None, the current shape will be maintained.
 
         """
-        # If data is None, ensure previous shape is maintained, and that it is
-        # not wrapped in an np.array
-        dataless = data is None
-        if dataless:
+        if dataless := data is None:
             self._shape = self.shape
+            self._lazy_array = None
+            self._real_array = None
 
         # Ensure we have numpy-like data.
-        elif not (hasattr(data, "shape") and hasattr(data, "dtype")):
-            data = np.asanyarray(data)
-
-        # Determine whether the class already has a defined shape,
-        # as this method is called from __init__.
-        has_shape = self.shape is not None
-        if has_shape and not dataless and self.shape != data.shape:
-            # The _ONLY_ data reshape permitted is converting a 0-dimensional
-            # array i.e. self.shape == () into a 1-dimensional array of length
-            # one i.e. data.shape == (1,)
-            if self.shape or data.shape != (1,):
-                emsg = "Require data with shape {!r}, got {!r}."
-                raise ValueError(emsg.format(self.shape, data.shape))
-
-        # Set lazy or real data, and reset the other.
-        if is_lazy_data(data):
-            self._lazy_array = data
-            self._real_array = None
         else:
-            if not ma.isMaskedArray(data):
-                # Coerce input data to ndarray (including ndarray subclasses).
-                if not dataless:
+            if not (hasattr(data, "shape") and hasattr(data, "dtype")):
+                data = np.asanyarray(data)
+
+            # Determine whether the class already has a defined shape,
+            # as this method is called from __init__.
+            has_shape = self.shape is not None
+            if has_shape and self.shape != data.shape:
+                # The _ONLY_ data reshape permitted is converting a 0-dimensional
+                # array i.e. self.shape == () into a 1-dimensional array of length
+                # one i.e. data.shape == (1,)
+                if self.shape or data.shape != (1,):
+                    emsg = "Require data with shape {!r}, got {!r}."
+                    raise ValueError(emsg.format(self.shape, data.shape))
+
+            # Set lazy or real data, and reset the other.
+            if is_lazy_data(data):
+                self._lazy_array = data
+                self._real_array = None
+            else:
+                if not ma.isMaskedArray(data):
+                    # Coerce input data to ndarray (including ndarray subclasses).
                     data = np.asarray(data)
-            if isinstance(data, ma.core.MaskedConstant):
-                # Promote to a masked array so that the fill-value is
-                # writeable to the data owner.
-                data = ma.array(data.data, mask=data.mask, dtype=data.dtype)
-            self._lazy_array = None
-            self._real_array = data
+                if isinstance(data, ma.core.MaskedConstant):
+                    # Promote to a masked array so that the fill-value is
+                    # writeable to the data owner.
+                    data = ma.array(data.data, mask=data.mask, dtype=data.dtype)
+                self._lazy_array = None
+                self._real_array = data
 
-        # Check the manager contract, as the managed data has changed.
+            # Check the manager contract, as the managed data has changed.
         self._assert_axioms()
 
     @property
@@ -292,21 +293,17 @@ def ndim(self):
     @property
     def shape(self):
         """The shape of the data being managed."""
-        if self.core_data() is None:
-            result = self._shape
-        else:
-            result = self.core_data().shape
-        return result
+        return self._shape if self._shape else self.core_data().shape
 
-    def is_dataless(self):
-        """Determine whether the cube is dataless.
+    def is_dataless(self) -> bool:
+        """Determine whether the cube has no data.
 
         Returns
         -------
         bool
 
         """
-        return (self.core_data() is None) and (self.shape is not None)
+        return self.core_data() is None
 
     def copy(self, data=None):
         """Return a deep copy of this :class:`~iris._data_manager.DataManager` instance.

From abb5a57a0a4f29cee81ea150a65f25b882003ec1 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 18 Dec 2024 17:27:12 +0000
Subject: [PATCH 38/48] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 lib/iris/__init__.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/iris/__init__.py b/lib/iris/__init__.py
index 64d1fe64c9..96095bb74a 100644
--- a/lib/iris/__init__.py
+++ b/lib/iris/__init__.py
@@ -143,6 +143,7 @@ def callback(cube, field, filename):
 #: To be used when copying a cube to make the new cube dataless.
 DATALESS = "NONE"
 
+
 class Future(threading.local):
     """Run-time configuration controller."""
 

From 94d02eac498a99570c404269cb4578abcda1fef6 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Wed, 18 Dec 2024 17:34:32 +0000
Subject: [PATCH 39/48] fixed broken review suggestion

---
 lib/iris/__init__.py      |  1 +
 lib/iris/_data_manager.py | 11 +++++++----
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/lib/iris/__init__.py b/lib/iris/__init__.py
index 64d1fe64c9..96095bb74a 100644
--- a/lib/iris/__init__.py
+++ b/lib/iris/__init__.py
@@ -143,6 +143,7 @@ def callback(cube, field, filename):
 #: To be used when copying a cube to make the new cube dataless.
 DATALESS = "NONE"
 
+
 class Future(threading.local):
     """Run-time configuration controller."""
 
diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 2268ec39f5..823c9afdba 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -193,11 +193,10 @@ def _deepcopy(self, memo, data=None):
     @property
     def data(self):
         """Return the real data. Any lazy data being managed will be realised.
-        ``None`` will be returned if in the dataless state.
 
         Returns
         -------
-        :class:`~numpy.ndarray` or :class:`numpy.ma.core.MaskedArray`.
+        :class:`~numpy.ndarray` or :class:`numpy.ma.core.MaskedArray` or None.
 
         """
         if self.has_lazy_data():
@@ -241,7 +240,7 @@ def data(self, data):
             managed. If data is None, the current shape will be maintained.
 
         """
-        if dataless := data is None:
+        if data is None:
             self._shape = self.shape
             self._lazy_array = None
             self._real_array = None
@@ -293,7 +292,11 @@ def ndim(self):
     @property
     def shape(self):
         """The shape of the data being managed."""
-        return self._shape if self._shape else self.core_data().shape
+        if self.core_data() is None:
+            result = self._shape
+        else:
+            result = self.core_data().shape
+        return result
 
     def is_dataless(self) -> bool:
         """Determine whether the cube has no data.

From 5c5bbf81ea10d87c8a887e20f80305d2343af831 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Fri, 20 Dec 2024 19:23:23 +0000
Subject: [PATCH 40/48] fixed some problems, and ensured self._shape is always
 set

---
 lib/iris/_data_manager.py                            | 12 +++++-------
 lib/iris/tests/unit/data_manager/test_DataManager.py |  2 +-
 2 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 823c9afdba..26a4750062 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -144,7 +144,7 @@ def _assert_axioms(self):
             if is_lazy and is_real:
                 msg = "Unexpected data state, got both lazy and real data."
                 raise ValueError(msg)
-            if not self._shape and not (is_lazy or is_real):
+            elif not self._shape:
                 msg = "Unexpected data state, got no lazy or real data, and no shape."
                 raise ValueError(msg)
 
@@ -252,7 +252,7 @@ def data(self, data):
 
             # Determine whether the class already has a defined shape,
             # as this method is called from __init__.
-            has_shape = self.shape is not None
+            has_shape = self._shape is not None
             if has_shape and self.shape != data.shape:
                 # The _ONLY_ data reshape permitted is converting a 0-dimensional
                 # array i.e. self.shape == () into a 1-dimensional array of length
@@ -275,6 +275,8 @@ def data(self, data):
                     data = ma.array(data.data, mask=data.mask, dtype=data.dtype)
                 self._lazy_array = None
                 self._real_array = data
+            if not has_shape:
+                self._shape = self.core_data().shape
 
             # Check the manager contract, as the managed data has changed.
         self._assert_axioms()
@@ -292,11 +294,7 @@ def ndim(self):
     @property
     def shape(self):
         """The shape of the data being managed."""
-        if self.core_data() is None:
-            result = self._shape
-        else:
-            result = self.core_data().shape
-        return result
+        return self._shape if self._shape else self.core_data().shape
 
     def is_dataless(self) -> bool:
         """Determine whether the cube has no data.
diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 83ba548653..878809f3d7 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -649,7 +649,7 @@ def test_with_lazy_array(self):
 class Test_is_dataless(tests.IrisTest):
     def setUp(self):
         self.data = np.array(0)
-        self.shape = 0
+        self.shape = (0, )
 
     def test_with_data(self):
         dm = DataManager(self.data)

From de348f96482b28a3a05846af1316932505b36fed Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Fri, 20 Dec 2024 19:24:13 +0000
Subject: [PATCH 41/48] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 lib/iris/tests/unit/data_manager/test_DataManager.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 878809f3d7..342f31637b 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -649,7 +649,7 @@ def test_with_lazy_array(self):
 class Test_is_dataless(tests.IrisTest):
     def setUp(self):
         self.data = np.array(0)
-        self.shape = (0, )
+        self.shape = (0,)
 
     def test_with_data(self):
         dm = DataManager(self.data)

From 7c74460d60799102a6c00d1ad3aceaf56cbc85a4 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Fri, 20 Dec 2024 19:57:46 +0000
Subject: [PATCH 42/48] fixed __eq__ and __repr__, and maybe dtype of dataless
 equal None

---
 lib/iris/_data_manager.py | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 26a4750062..4ce74ff23f 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -94,12 +94,14 @@ def __eq__(self, other):
         result = NotImplemented
 
         if isinstance(other, type(self)):
-            result = False
-            same_lazy = self.has_lazy_data() == other.has_lazy_data()
-            same_dtype = self.dtype == other.dtype
-            if same_lazy and same_dtype:
-                result = array_equal(self.core_data(), other.core_data())
-
+            if self.is_dataless() and other.is_dataless():
+                result = self.shape == other.shape
+            else:
+                result = False
+                same_lazy = self.has_lazy_data() == other.has_lazy_data()
+                same_dtype = self.dtype == other.dtype
+                if same_lazy and same_dtype:
+                    result = array_equal(self.core_data(), other.core_data())
         return result
 
     def __ne__(self, other):
@@ -131,6 +133,8 @@ def __repr__(self):
         """Return an string representation of the instance."""
         fmt = "{cls}({data!r})"
         result = fmt.format(data=self.core_data(), cls=type(self).__name__)
+        if self.is_dataless():
+            result = f"{result}, shape={self.shape}"
 
         return result
 
@@ -284,7 +288,7 @@ def data(self, data):
     @property
     def dtype(self):
         """The dtype of the realised lazy data or the dtype of the real data."""
-        return self.core_data().dtype
+        return self.core_data().dtype if not self.is_dataless() else None
 
     @property
     def ndim(self):
@@ -294,7 +298,7 @@ def ndim(self):
     @property
     def shape(self):
         """The shape of the data being managed."""
-        return self._shape if self._shape else self.core_data().shape
+        return self._shape
 
     def is_dataless(self) -> bool:
         """Determine whether the cube has no data.
@@ -365,7 +369,9 @@ def lazy_data(self):
             This method will never realise any lazy data.
 
         """
-        if self.has_lazy_data():
+        if self.is_dataless():
+            result = None
+        elif self.has_lazy_data():
             result = self._lazy_array
         else:
             result = as_lazy_data(self._real_array)

From 8bddec761eaa5b9df69390a1ffeb8b69767767df Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Mon, 23 Dec 2024 17:29:23 +0000
Subject: [PATCH 43/48] included self.core_data().shape when shape is none, for
 Coord cases

---
 lib/iris/_data_manager.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 4ce74ff23f..7047da69b9 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -298,7 +298,7 @@ def ndim(self):
     @property
     def shape(self):
         """The shape of the data being managed."""
-        return self._shape
+        return self._shape if self._shape else self.core_data().shape
 
     def is_dataless(self) -> bool:
         """Determine whether the cube has no data.

From 2a6705013c359b0d9b99be3d98e2402cf3b10ee6 Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Tue, 24 Dec 2024 19:21:23 +0000
Subject: [PATCH 44/48] made sure _shape is set in case of prexisting _shape of
 ()

---
 lib/iris/_data_manager.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index 7047da69b9..ecfafabe62 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -279,7 +279,9 @@ def data(self, data):
                     data = ma.array(data.data, mask=data.mask, dtype=data.dtype)
                 self._lazy_array = None
                 self._real_array = data
-            if not has_shape:
+            # sets ``self._shape`` if it is None, or if it is being converted from
+            # ( ) to (1, )
+            if not has_shape or (self._shape == () and data.shape == (1,)):
                 self._shape = self.core_data().shape
 
             # Check the manager contract, as the managed data has changed.

From f8c2daddf44424ae7683b2ca614200d40b21ddbc Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Tue, 24 Dec 2024 20:21:47 +0000
Subject: [PATCH 45/48] added eq and repr tests, and fixed shape of () not
 being valid

---
 lib/iris/_data_manager.py                       |  4 ++--
 .../tests/unit/data_manager/test_DataManager.py | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index ecfafabe62..ea044cc180 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -148,7 +148,7 @@ def _assert_axioms(self):
             if is_lazy and is_real:
                 msg = "Unexpected data state, got both lazy and real data."
                 raise ValueError(msg)
-            elif not self._shape:
+            elif self._shape is None:
                 msg = "Unexpected data state, got no lazy or real data, and no shape."
                 raise ValueError(msg)
 
@@ -300,7 +300,7 @@ def ndim(self):
     @property
     def shape(self):
         """The shape of the data being managed."""
-        return self._shape if self._shape else self.core_data().shape
+        return self._shape
 
     def is_dataless(self) -> bool:
         """Determine whether the cube has no data.
diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 342f31637b..f9ee73bc18 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -87,6 +87,16 @@ def test_lazy_with_lazy__dtype_failure(self):
         dm2 = DataManager(as_lazy_data(self.real_array).astype(int))
         self.assertFalse(dm1 == dm2)
 
+    def test_none(self):
+        dm1 = DataManager(data=None, shape=(1, ))
+        dm2 = DataManager(data=None, shape=(1, ))
+        self.assertTrue(dm1 == dm2)
+
+    def test_none_with_real(self):
+        dm1 = DataManager(data=None, shape=(1, ))
+        dm2 = DataManager(self.real_array)
+        self.assertFalse(dm1 == dm2)
+
     def test_non_DataManager_failure(self):
         dm = DataManager(np.array(0))
         self.assertFalse(dm == 0)
@@ -158,6 +168,13 @@ def test_lazy(self):
         expected = "{}({!r})".format(self.name, self.lazy_array)
         self.assertEqual(result, expected)
 
+    def test_dataless(self):
+        print(self.real_array.shape)
+        dm = DataManager(None, self.real_array.shape)
+        result = repr(dm)
+        expected = "{}({!r}), shape={}".format(self.name, None, self.real_array.shape)
+        self.assertEqual(result, expected)
+
 
 class Test__assert_axioms(tests.IrisTest):
     def setUp(self):

From ed75485e857de426b86f116c92bf1643fa8c5bd7 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 24 Dec 2024 20:22:30 +0000
Subject: [PATCH 46/48] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 lib/iris/tests/unit/data_manager/test_DataManager.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index f9ee73bc18..58004b120e 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -88,12 +88,12 @@ def test_lazy_with_lazy__dtype_failure(self):
         self.assertFalse(dm1 == dm2)
 
     def test_none(self):
-        dm1 = DataManager(data=None, shape=(1, ))
-        dm2 = DataManager(data=None, shape=(1, ))
+        dm1 = DataManager(data=None, shape=(1,))
+        dm2 = DataManager(data=None, shape=(1,))
         self.assertTrue(dm1 == dm2)
 
     def test_none_with_real(self):
-        dm1 = DataManager(data=None, shape=(1, ))
+        dm1 = DataManager(data=None, shape=(1,))
         dm2 = DataManager(self.real_array)
         self.assertFalse(dm1 == dm2)
 

From 922191779a1da9d5423971127d18f4314b3cf7ba Mon Sep 17 00:00:00 2001
From: Elias Sadek <elias.sadek@metoffice.gov.uk>
Date: Tue, 24 Dec 2024 20:30:32 +0000
Subject: [PATCH 47/48] fixed test now that self._shape is always set

---
 lib/iris/tests/unit/data_manager/test_DataManager.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 58004b120e..8e6df63158 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -184,6 +184,7 @@ def setUp(self):
 
     def test_array_none(self):
         self.dm._real_array = None
+        self.dm._shape = None
         emsg = "Unexpected data state, got no lazy or real data, and no shape."
         with self.assertRaisesRegex(ValueError, emsg):
             self.dm._assert_axioms()

From f8c5d9512341f406ca831d170b778385dd625b14 Mon Sep 17 00:00:00 2001
From: Elias <110238618+ESadek-MO@users.noreply.github.com>
Date: Fri, 7 Feb 2025 13:33:52 +0000
Subject: [PATCH 48/48] move `self._shape = shape` to init block

---
 lib/iris/_data_manager.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/lib/iris/_data_manager.py b/lib/iris/_data_manager.py
index ea044cc180..6a0297adcd 100644
--- a/lib/iris/_data_manager.py
+++ b/lib/iris/_data_manager.py
@@ -38,9 +38,8 @@ def __init__(self, data, shape=None):
             msg = '"shape" should only be provided if "data" is None'
             raise ValueError(msg)
 
-        self._shape = shape
-
         # Initialise the instance.
+        self._shape = shape
         self._lazy_array = None
         self._real_array = None