Fix unnecessary copy in strings_to_categoricals (#298)

Removed an explicit `init_as_actual` in `strings_to_categoricals` in favour the implicit one from modifying the dataframe.

Split the raw r/w test into two (actualized AnnData and view), and check if the warning still occurs in the view one.

Co-authored-by: Philipp A. <[email protected]>

anndata/_core/anndata.py

@@ -1143,14 +1143,8 @@ def strings_to_categoricals(self, df: Optional[pd.DataFrame] = None):
         dont_modify = False  # only necessary for backed views
         if df is None:
             dfs = [self.obs, self.var]
-            if self.is_view:
-                if not self.isbacked:
-                    warnings.warn(
-                        "Initializing view as actual.", ImplicitModificationWarning,
-                    )
-                    self._init_as_actual(self.copy())
-                else:
-                    dont_modify = True
+            if self.is_view and self.isbacked:
+                dont_modify = True
         else:
             dfs = [df]
         for df in dfs:
@@ -1173,6 +1167,12 @@ def strings_to_categoricals(self, df: Optional[pd.DataFrame] = None):
                         "AnnData, not on this view. You might encounter this"
                         "error message while copying or writing to disk."
                     )
+                if self.is_view:
+                    warnings.warn(
+                        "Initializing view as actual.", ImplicitModificationWarning
+                    )
+                # If `self` is a view, it will be actualized in the next line,
+                # therefore the previous warning
                 df[key] = c
                 logger.info(f"... storing {key!r} as categorical")
 

anndata/tests/test_raw.py

@@ -3,6 +3,7 @@
 
 import anndata as ad
 from anndata._core.anndata import ImplicitModificationWarning
+from anndata.tests.helpers import assert_equal
 
 
 # -------------------------------------------------------------------------------
@@ -37,7 +38,9 @@ def adata_raw():
         np.array(data), obs=obs_dict, var=var_dict, uns=uns_dict, dtype="int32"
     )
     adata.raw = adata
-    return adata[:, [0, 1]]
+    # Make them different shapes
+    adata = adata[:, [0, 1]].copy()
+    return adata
 
 
 # -------------------------------------------------------------------------------
@@ -65,22 +68,43 @@ def test_raw_of_view(adata_raw):
 
 
 def test_raw_rw(adata_raw, backing_h5ad):
-    with pytest.warns(
-        ImplicitModificationWarning, match="Initializing view as actual"
-    ):  # TODO: don’t modify adata just to write it
-        adata_raw.write(backing_h5ad)
-    adata_raw = ad.read(backing_h5ad)
+    adata_raw.write(backing_h5ad)
+    adata_read = ad.read(backing_h5ad)
+
+    assert_equal(adata_read, adata_raw, exact=True)
+
+    assert adata_raw.var_names.tolist() == ["var1", "var2"]
+    assert adata_raw.raw.var_names.tolist() == ["var1", "var2", "var3"]
+    assert adata_raw.raw[:, 0].X.tolist() == [[1], [4], [7]]
+
+
+def test_raw_view_rw(adata_raw, backing_h5ad):
+    # Make sure it still writes correctly if the object is a view
+    adata_raw_view = adata_raw[:, adata_raw.var_names]
+    assert_equal(adata_raw_view, adata_raw)
+    with pytest.warns(ImplicitModificationWarning, match="Initializing view as actual"):
+        adata_raw_view.write(backing_h5ad)
+    adata_read = ad.read(backing_h5ad)
+
+    assert_equal(adata_read, adata_raw_view, exact=True)
 
     assert adata_raw.var_names.tolist() == ["var1", "var2"]
     assert adata_raw.raw.var_names.tolist() == ["var1", "var2", "var3"]
     assert adata_raw.raw[:, 0].X.tolist() == [[1], [4], [7]]
 
 
 def test_raw_backed(adata_raw, backing_h5ad):
-    with pytest.warns(
-        ImplicitModificationWarning, match="Initializing view as actual"
-    ):  # TODO: don’t modify adata just to write it
-        adata_raw.filename = backing_h5ad
+    adata_raw.filename = backing_h5ad
+
+    assert adata_raw.var_names.tolist() == ["var1", "var2"]
+    assert adata_raw.raw.var_names.tolist() == ["var1", "var2", "var3"]
+    if adata_raw.raw[:, 0].X.shape[1] != 1:
+        pytest.xfail("Raw is broken for backed slices")
+    assert adata_raw.raw[:, 0].X[:].tolist() == [[1], [4], [7]]
+
+
+def test_raw_view_backed(adata_raw, backing_h5ad):
+    adata_raw.filename = backing_h5ad
 
     assert adata_raw.var_names.tolist() == ["var1", "var2"]
     assert adata_raw.raw.var_names.tolist() == ["var1", "var2", "var3"]