Fixed #20867 -- Added the Form.add_error() method.

Refs #20199 #16986.

Thanks @akaariai, @bmispelon, @mjtamlyn, @timgraham for the reviews.

django/core/exceptions.py

@@ -77,64 +77,78 @@ class ValidationError(Exception):
     """An error while validating data."""
     def __init__(self, message, code=None, params=None):
         """
-        ValidationError can be passed any object that can be printed (usually
+        The `message` argument can be a single error, a list of errors, or a
-        a string), a list of objects or a dictionary.
+        dictionary that maps field names to lists of errors. What we define as
+        an "error" can be either a simple string or an instance of
+        ValidationError with its message attribute set, and what we define as
+        list or dictionary can be an actual `list` or `dict` or an instance
+        of ValidationError with its `error_list` or `error_dict` attribute set.
         """
-        if isinstance(message, dict):
+        if isinstance(message, ValidationError):
-            self.error_dict = message
+            if hasattr(message, 'error_dict'):
-        elif isinstance(message, list):
+                message = message.error_dict
-            self.error_list = message
+            elif not hasattr(message, 'message'):
+                message = message.error_list
             else:
+                message, code, params = message.message, message.code, message.params
+
+        if isinstance(message, dict):
+            self.error_dict = {}
+            for field, messages in message.items():
+                if not isinstance(messages, ValidationError):
+                    messages = ValidationError(messages)
+                self.error_dict[field] = messages.error_list
+
+        elif isinstance(message, list):
+            self.error_list = []
+            for message in message:
+                # Normalize plain strings to instances of ValidationError.
+                if not isinstance(message, ValidationError):
+                    message = ValidationError(message)
+                self.error_list.extend(message.error_list)
+
+        else:
+            self.message = message
             self.code = code
             self.params = params
-            self.message = message
             self.error_list = [self]
 
     @property
     def message_dict(self):
-        message_dict = {}
+        return dict(self)
-        for field, messages in self.error_dict.items():
-            message_dict[field] = []
-            for message in messages:
-                if isinstance(message, ValidationError):
-                    message_dict[field].extend(message.messages)
-                else:
-                    message_dict[field].append(force_text(message))
-        return message_dict
 
     @property
     def messages(self):
         if hasattr(self, 'error_dict'):
-            message_list = reduce(operator.add, self.error_dict.values())
+            return reduce(operator.add, dict(self).values())
-        else:
+        return list(self)
-            message_list = self.error_list
-
-        messages = []
-        for message in message_list:
-            if isinstance(message, ValidationError):
-                params = message.params
-                message = message.message
-                if params:
-                    message %= params
-            message = force_text(message)
-            messages.append(message)
-        return messages
-
-    def __str__(self):
-        if hasattr(self, 'error_dict'):
-            return repr(self.message_dict)
-        return repr(self.messages)
-
-    def __repr__(self):
-        return 'ValidationError(%s)' % self
 
     def update_error_dict(self, error_dict):
         if hasattr(self, 'error_dict'):
             if error_dict:
-                for k, v in self.error_dict.items():
+                for field, errors in self.error_dict.items():
-                    error_dict.setdefault(k, []).extend(v)
+                    error_dict.setdefault(field, []).extend(errors)
             else:
                 error_dict = self.error_dict
         else:
             error_dict[NON_FIELD_ERRORS] = self.error_list
         return error_dict
+
+    def __iter__(self):
+        if hasattr(self, 'error_dict'):
+            for field, errors in self.error_dict.items():
+                yield field, list(ValidationError(errors))
+        else:
+            for error in self.error_list:
+                message = error.message
+                if error.params:
+                    message %= error.params
+                yield force_text(message)
+
+    def __str__(self):
+        if hasattr(self, 'error_dict'):
+            return repr(dict(self))
+        return repr(list(self))
+
+    def __repr__(self):
+        return 'ValidationError(%s)' % self

django/db/models/base.py

@@ -987,7 +987,7 @@ class Model(six.with_metaclass(ModelBase)):
 
     def clean_fields(self, exclude=None):
         """
-        Cleans all fields and raises a ValidationError containing message_dict
+        Cleans all fields and raises a ValidationError containing a dict
         of all validation errors if any occur.
         """
         if exclude is None:

django/forms/forms.py

@@ -290,6 +290,51 @@ class BaseForm(object):
         prefix = self.add_prefix(fieldname)
         return field.widget.value_from_datadict(self.data, self.files, prefix)
 
+    def add_error(self, field, error):
+        """
+        Update the content of `self._errors`.
+
+        The `field` argument is the name of the field to which the errors
+        should be added. If its value is None the errors will be treated as
+        NON_FIELD_ERRORS.
+
+        The `error` argument can be a single error, a list of errors, or a
+        dictionary that maps field names to lists of errors. What we define as
+        an "error" can be either a simple string or an instance of
+        ValidationError with its message attribute set and what we define as
+        list or dictionary can be an actual `list` or `dict` or an instance
+        of ValidationError with its `error_list` or `error_dict` attribute set.
+
+        If `error` is a dictionary, the `field` argument *must* be None and
+        errors will be added to the fields that correspond to the keys of the
+        dictionary.
+        """
+        if not isinstance(error, ValidationError):
+            # Normalize to ValidationError and let its constructor
+            # do the hard work of making sense of the input.
+            error = ValidationError(error)
+
+        if hasattr(error, 'error_dict'):
+            if field is not None:
+                raise TypeError(
+                    "The argument `field` must be `None` when the `error` "
+                    "argument contains errors for multiple fields."
+                )
+            else:
+                error = dict(error)
+        else:
+            error = {field or NON_FIELD_ERRORS: list(error)}
+
+        for field, error_list in error.items():
+            if field not in self.errors:
+                if field != NON_FIELD_ERRORS and field not in self.fields:
+                    raise ValueError(
+                        "'%s' has no field named '%s'." % (self.__class__.__name__, field))
+                self._errors[field] = self.error_class()
+            self._errors[field].extend(error_list)
+            if field in self.cleaned_data:
+                del self.cleaned_data[field]
+
     def full_clean(self):
         """
         Cleans all of self.data and populates self._errors and
@@ -303,6 +348,7 @@ class BaseForm(object):
         # changed from the initial data, short circuit any validation.
         if self.empty_permitted and not self.has_changed():
             return
+
         self._clean_fields()
         self._clean_form()
         self._post_clean()
@@ -324,15 +370,13 @@ class BaseForm(object):
                     value = getattr(self, 'clean_%s' % name)()
                     self.cleaned_data[name] = value
             except ValidationError as e:
-                self._errors[name] = self.error_class(e.messages)
+                self.add_error(name, e)
-                if name in self.cleaned_data:
-                    del self.cleaned_data[name]
 
     def _clean_form(self):
         try:
             cleaned_data = self.clean()
         except ValidationError as e:
-            self._errors[NON_FIELD_ERRORS] = self.error_class(e.messages)
+            self.add_error(None, e)
         else:
             if cleaned_data is not None:
                 self.cleaned_data = cleaned_data

django/forms/models.py

@@ -326,27 +326,6 @@ class BaseModelForm(BaseForm):
         super(BaseModelForm, self).__init__(data, files, auto_id, prefix, object_data,
                                             error_class, label_suffix, empty_permitted)
 
-    def _update_errors(self, errors):
-        for field, messages in errors.error_dict.items():
-            if field not in self.fields:
-                continue
-            field = self.fields[field]
-            for message in messages:
-                if isinstance(message, ValidationError):
-                    if message.code in field.error_messages:
-                        message.message = field.error_messages[message.code]
-
-        message_dict = errors.message_dict
-        for k, v in message_dict.items():
-            if k != NON_FIELD_ERRORS:
-                self._errors.setdefault(k, self.error_class()).extend(v)
-                # Remove the data from the cleaned_data dict since it was invalid
-                if k in self.cleaned_data:
-                    del self.cleaned_data[k]
-        if NON_FIELD_ERRORS in message_dict:
-            messages = message_dict[NON_FIELD_ERRORS]
-            self._errors.setdefault(NON_FIELD_ERRORS, self.error_class()).extend(messages)
-
     def _get_validation_exclusions(self):
         """
         For backwards-compatibility, several types of fields need to be
@@ -393,6 +372,20 @@ class BaseModelForm(BaseForm):
         self._validate_unique = True
         return self.cleaned_data
 
+    def _update_errors(self, errors):
+        # Override any validation error messages defined at the model level
+        # with those defined on the form fields.
+        for field, messages in errors.error_dict.items():
+            if field not in self.fields:
+                continue
+            field = self.fields[field]
+            for message in messages:
+                if (isinstance(message, ValidationError) and
+                        message.code in field.error_messages):
+                    message.message = field.error_messages[message.code]
+
+        self.add_error(None, errors)
+
     def _post_clean(self):
         opts = self._meta
         # Update the model instance with self.cleaned_data.
@@ -407,13 +400,12 @@ class BaseModelForm(BaseForm):
         # object being referred to may not yet fully exist (#12749).
         # However, these fields *must* be included in uniqueness checks,
         # so this can't be part of _get_validation_exclusions().
-        for f_name, field in self.fields.items():
+        for name, field in self.fields.items():
             if isinstance(field, InlineForeignKeyField):
-                exclude.append(f_name)
+                exclude.append(name)
 
         try:
-            self.instance.full_clean(exclude=exclude,
+            self.instance.full_clean(exclude=exclude, validate_unique=False)
-                validate_unique=False)
         except ValidationError as e:
             self._update_errors(e)
 
@@ -695,6 +687,7 @@ class BaseModelFormSet(BaseFormSet):
                         del form.cleaned_data[field]
                     # mark the data as seen
                     seen_data.add(data)
+
         if errors:
             raise ValidationError(errors)
 

docs/ref/forms/api.txt

@@ -117,6 +117,26 @@ The validation routines will only get called once, regardless of how many times
 you access :attr:`~Form.errors` or call :meth:`~Form.is_valid`. This means that
 if validation has side effects, those side effects will only be triggered once.
 
+.. method:: Form.add_error(field, error)
+
+.. versionadded:: 1.7
+
+This method allows adding errors to specific fields from within the
+``Form.clean()`` method, or from outside the form altogether; for instance
+from a view. This is a better alternative to fiddling directly with
+``Form._errors`` as described in :ref:`modifying-field-errors`.
+
+The ``field`` argument is the name of the field to which the errors
+should be added. If its value is ``None`` the error will be treated as
+a non-field error as returned by ``Form.non_field_errors()``.
+
+The ``error`` argument can be a simple string, or preferably an instance of
+``ValidationError``. See :ref:`raising-validation-error` for best practices
+when defining form errors.
+
+Note that ``Form.add_error()`` automatically removes the relevant field from
+``cleaned_data``.
+
 Behavior of unbound forms
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 

docs/ref/forms/validation.txt

@@ -464,3 +464,31 @@ Secondly, once we have decided that the combined data in the two fields we are
 considering aren't valid, we must remember to remove them from the
 ``cleaned_data``. `cleaned_data`` is present even if the form doesn't
 validate, but it contains only field values that did validate.
+
+.. versionchanged:: 1.7
+
+In lieu of manipulating ``_errors`` directly, it's now possible to add errors
+to specific fields with :meth:`django.forms.Form.add_error()`::
+
+    from django import forms
+
+    class ContactForm(forms.Form):
+        # Everything as before.
+        ...
+
+        def clean(self):
+            cleaned_data = super(ContactForm, self).clean()
+            cc_myself = cleaned_data.get("cc_myself")
+            subject = cleaned_data.get("subject")
+
+            if cc_myself and subject and "help" not in subject:
+                msg = u"Must put 'help' in subject when cc'ing yourself."
+                self.add_error('cc_myself', msg)
+                self.add_error('subject', msg)
+
+The second argument of ``add_error()`` can be a simple string, or preferably
+an instance of ``ValidationError``. See :ref:`raising-validation-error` for
+more details.
+
+Unlike the ``_errors`` approach, ``add_error()` automatically removes the field
+from ``cleaned_data``.

docs/releases/1.7.txt

@@ -350,6 +350,9 @@ Forms
 * It's now possible to opt-out from a ``Form`` field declared in a parent class
   by shadowing it with a non-``Field`` value.
 
+* The new :meth:`~django.forms.Form.add_error()` method allows adding errors
+  to specific form fields.
+
 Internationalization
 ^^^^^^^^^^^^^^^^^^^^
 

tests/forms_tests/tests/test_forms.py

@@ -657,25 +657,49 @@ class FormsTestCase(TestCase):
         self.assertEqual(f.cleaned_data['password2'], 'foo')
 
         # Another way of doing multiple-field validation is by implementing the
-        # Form's clean() method. If you do this, any ValidationError raised by that
+        # Form's clean() method. Usually ValidationError raised by that method
-        # method will not be associated with a particular field; it will have a
+        # will not be associated with a particular field and will have a
-        # special-case association with the field named '__all__'.
+        # special-case association with the field named '__all__'. It's
-        # Note that in Form.clean(), you have access to self.cleaned_data, a dictionary of
+        # possible to associate the errors to particular field with the
-        # all the fields/values that have *not* raised a ValidationError. Also note
+        # Form.add_error() method or by passing a dictionary that maps each
-        # Form.clean() is required to return a dictionary of all clean data.
+        # field to one or more errors.
+        #
+        # Note that in Form.clean(), you have access to self.cleaned_data, a
+        # dictionary of all the fields/values that have *not* raised a
+        # ValidationError. Also note Form.clean() is required to return a
+        # dictionary of all clean data.
         class UserRegistration(Form):
             username = CharField(max_length=10)
             password1 = CharField(widget=PasswordInput)
             password2 = CharField(widget=PasswordInput)
 
             def clean(self):
+                # Test raising a ValidationError as NON_FIELD_ERRORS.
                 if self.cleaned_data.get('password1') and self.cleaned_data.get('password2') and self.cleaned_data['password1'] != self.cleaned_data['password2']:
                     raise ValidationError('Please make sure your passwords match.')
 
+                # Test raising ValidationError that targets multiple fields.
+                errors = {}
+                if self.cleaned_data.get('password1') == 'FORBIDDEN_VALUE':
+                    errors['password1'] = 'Forbidden value.'
+                if self.cleaned_data.get('password2') == 'FORBIDDEN_VALUE':
+                    errors['password2'] = ['Forbidden value.']
+                if errors:
+                    raise ValidationError(errors)
+
+                # Test Form.add_error()
+                if self.cleaned_data.get('password1') == 'FORBIDDEN_VALUE2':
+                    self.add_error(None, 'Non-field error 1.')
+                    self.add_error('password1', 'Forbidden value 2.')
+                if self.cleaned_data.get('password2') == 'FORBIDDEN_VALUE2':
+                    self.add_error('password2', 'Forbidden value 2.')
+                    raise ValidationError('Non-field error 2.')
+
                 return self.cleaned_data
 
         f = UserRegistration(auto_id=False)
         self.assertEqual(f.errors, {})
+
         f = UserRegistration({}, auto_id=False)
         self.assertHTMLEqual(f.as_table(), """<tr><th>Username:</th><td><ul class="errorlist"><li>This field is required.</li></ul><input type="text" name="username" maxlength="10" /></td></tr>
 <tr><th>Password1:</th><td><ul class="errorlist"><li>This field is required.</li></ul><input type="password" name="password1" /></td></tr>
@@ -683,6 +707,7 @@ class FormsTestCase(TestCase):
         self.assertEqual(f.errors['username'], ['This field is required.'])
         self.assertEqual(f.errors['password1'], ['This field is required.'])
         self.assertEqual(f.errors['password2'], ['This field is required.'])
+
         f = UserRegistration({'username': 'adrian', 'password1': 'foo', 'password2': 'bar'}, auto_id=False)
         self.assertEqual(f.errors['__all__'], ['Please make sure your passwords match.'])
         self.assertHTMLEqual(f.as_table(), """<tr><td colspan="2"><ul class="errorlist"><li>Please make sure your passwords match.</li></ul></td></tr>
@@ -693,12 +718,25 @@ class FormsTestCase(TestCase):
 <li>Username: <input type="text" name="username" value="adrian" maxlength="10" /></li>
 <li>Password1: <input type="password" name="password1" /></li>
 <li>Password2: <input type="password" name="password2" /></li>""")
+
         f = UserRegistration({'username': 'adrian', 'password1': 'foo', 'password2': 'foo'}, auto_id=False)
         self.assertEqual(f.errors, {})
         self.assertEqual(f.cleaned_data['username'], 'adrian')
         self.assertEqual(f.cleaned_data['password1'], 'foo')
         self.assertEqual(f.cleaned_data['password2'], 'foo')
 
+        f = UserRegistration({'username': 'adrian', 'password1': 'FORBIDDEN_VALUE', 'password2': 'FORBIDDEN_VALUE'}, auto_id=False)
+        self.assertEqual(f.errors['password1'], ['Forbidden value.'])
+        self.assertEqual(f.errors['password2'], ['Forbidden value.'])
+
+        f = UserRegistration({'username': 'adrian', 'password1': 'FORBIDDEN_VALUE2', 'password2': 'FORBIDDEN_VALUE2'}, auto_id=False)
+        self.assertEqual(f.errors['__all__'], ['Non-field error 1.', 'Non-field error 2.'])
+        self.assertEqual(f.errors['password1'], ['Forbidden value 2.'])
+        self.assertEqual(f.errors['password2'], ['Forbidden value 2.'])
+
+        with six.assertRaisesRegex(self, ValueError, "has no field named"):
+            f.add_error('missing_field', 'Some error.')
+
     def test_dynamic_construction(self):
         # It's possible to construct a Form dynamically by adding to the self.fields
         # dictionary in __init__(). Don't forget to call Form.__init__() within the