Reference values in error messages. Fixes keleshev#4.

Author: Rogdham

README.rst

@@ -243,19 +243,22 @@ User-friendly error reporting
 
 You can pass a keyword argument ``error`` to any of validatable classes
 (such as ``Schema``, ``And``, ``Or``, ``Use``) to report this error instead of
-a built-in one.
+a built-in one. The format ``{value}`` will get replaced with the value failing
+the validation.
 
 .. code:: python
 
-    >>> Schema(Use(int, error='Invalid year')).validate('XVII')
+    >>> Schema(Use(int, error='Invalid year: {value}')).validate('XVII')
     Traceback (most recent call last):
     ...
-    SchemaError: Invalid year
+    SchemaError: Invalid year: XVII
 
 You can see all errors that occured by accessing exception's ``exc.autos``
 for auto-generated error messages, and ``exc.errors`` for errors
 which had ``error`` text passed to them.
 
+You can use ``exc.value`` to directly access the faulty value.
+
 You can exit with ``sys.exit(exc.code)`` if you want to show the messages
 to the user without traceback. ``error`` messages are given precedence in that
 case.

schema.py

@@ -9,9 +9,12 @@ class SchemaError(Exception):
 
     """Error during Schema validation."""
 
-    def __init__(self, autos, errors):
+    def __init__(self, autos, errors, value):
         self.autos = autos if type(autos) is list else [autos]
         self.errors = errors if type(errors) is list else [errors]
+        self.value = value
+        self.errors = [None if e is None else e.format(value=self.value)
+                       for e in self.errors]
         Exception.__init__(self, self.code)
 
     @property
@@ -47,14 +50,14 @@ def validate(self, data):
 class Or(And):
 
     def validate(self, data):
-        x = SchemaError([], [])
+        x = SchemaError([], [], None)
         for s in [Schema(s, error=self._error) for s in self._args]:
             try:
                 return s.validate(data)
             except SchemaError as _x:
                 x = _x
         raise SchemaError(['%r did not validate %r' % (self, data)] + x.autos,
-                         [self._error] + x.errors)
+                         [self._error] + x.errors, data)
 
 
 class Use(object):
@@ -71,10 +74,11 @@ def validate(self, data):
         try:
             return self._callable(data)
         except SchemaError as x:
-            raise SchemaError([None] + x.autos, [self._error] + x.errors)
+            raise SchemaError([None] + x.autos, [self._error] + x.errors, data)
         except BaseException as x:
             f = self._callable.__name__
-            raise SchemaError('%s(%r) raised %r' % (f, data, x), self._error)
+            raise SchemaError('%s(%r) raised %r' % (f, data, x), self._error,
+                              data)
 
 
 def priority(s):
@@ -118,9 +122,9 @@ def validate(self, data):
                 for skey in sorted(s, key=priority):
                     svalue = s[skey]
                     try:
-                        nkey = Schema(skey, error=e).validate(key)
+                        nkey = Schema(skey).validate(key) # error got forgotten
                         try:
-                            nvalue = Schema(svalue, error=e).validate(value)
+                            nvalue = Schema(svalue).validate(value)
                         except SchemaError as _x:
                             x = _x
                             raise
@@ -135,44 +139,47 @@ def validate(self, data):
                 elif skey is not None:
                     if x is not None:
                         raise SchemaError(['key %r is required' % key] +
-                                          x.autos, [e] + x.errors)
+                                          x.autos, [e] + x.errors, data)
                     else:
-                        raise SchemaError('key %r is required' % skey, e)
+                        raise SchemaError('key %r is required' % skey, e, data)
             coverage = set(k for k in coverage if type(k) is not Optional)
             required = set(k for k in s if type(k) is not Optional)
             if coverage != required:
-                raise SchemaError('missed keys %r' % (required - coverage), e)
+                raise SchemaError('missed keys %r' % (required - coverage), e,
+                                  data)
             if len(new) != len(data):
-                raise SchemaError('wrong keys %r in %r' % (new, data), e)
+                raise SchemaError('wrong keys %r in %r' % (new, data), e, data)
             return new
         if hasattr(s, 'validate'):
             try:
                 return s.validate(data)
             except SchemaError as x:
-                raise SchemaError([None] + x.autos, [e] + x.errors)
+                raise SchemaError([None] + x.autos, [e] + x.errors, data)
             except BaseException as x:
                 raise SchemaError('%r.validate(%r) raised %r' % (s, data, x),
-                                 self._error)
+                                  self._error, data)
         if type(s) is type:
             if isinstance(data, s):
                 return data
             else:
-                raise SchemaError('%r should be instance of %r' % (data, s), e)
+                raise SchemaError('%r should be instance of %r' % (data, s), e,
+                                  data)
         if callable(s):
             f = s.__name__
             try:
                 if s(data):
                     return data
             except SchemaError as x:
-                raise SchemaError([None] + x.autos, [e] + x.errors)
+                raise SchemaError([None] + x.autos, [e] + x.errors, data)
             except BaseException as x:
                 raise SchemaError('%s(%r) raised %r' % (f, data, x),
-                                  self._error)
-            raise SchemaError('%s(%r) should evaluate to True' % (f, data), e)
+                                  self._error, data)
+            raise SchemaError('%s(%r) should evaluate to True' % (f, data), e,
+                              data)
         if s == data:
             return data
         else:
-            raise SchemaError('%r does not match %r' % (s, data), e)
+            raise SchemaError('%r does not match %r' % (s, data), e, data)
 
 
 class Optional(Schema):

test_schema.py

@@ -19,8 +19,8 @@ def ve(_):
     raise ValueError()
 
 
-def se(_):
-    raise SchemaError('first auto', 'first error')
+def se(data):
+    raise SchemaError('first auto', 'first error', data)
 
 
 def test_schema():
@@ -164,6 +164,21 @@ def test_nice_errors():
         assert e.code == 'should be a number'
 
 
+def test_nice_errors_format():
+    try:
+        Schema(int, error='{value} should be integer').validate('x')
+    except SchemaError as e:
+        assert e.errors == ['x should be integer']
+    try:
+        Schema(Use(float), error='{value} should be a number').validate('x')
+    except SchemaError as e:
+        assert e.code == 'x should be a number'
+    try:
+        Schema({Optional('i'): Use(int, error='{value} should be a number')}).validate({'i': 'x'})
+    except SchemaError as e:
+        assert e.code == 'x should be a number'
+
+
 def test_use_error_handling():
     try:
         Use(ve).validate('x')