ValidationError has no attribute `error_list` if message is a dict, but Field.run_validators() depends on it

[Michael Käufl]

If the message is a dict, ValidationError has no attribute error_list:

class ValidationError(Exception):

    def __init__(self, message, code=None, params=None):
        # …
        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 = …
            # …
        else:
            # …
            self.error_list = [self]

See

• ​https://github.com/django/django/blob/2.0.4/django/core/exceptions.py#L115-L137
• ​https://github.com/django/django/blob/c3055242c81812278ebdc93dd109f30d2cbd1610/django/core/exceptions.py#L115-L137 (current master)

But Field.run_validators() depends on ValidationError having an attribute error_list:

class Field(RegisterLookupMixin):

    def run_validators(self, value):
        # …
        for v in self.validators:
            try:
                v(value)
            except exceptions.ValidationError as e:
                if hasattr(e, 'code') and e.code in self.error_messages:
                    e.message = self.error_messages[e.code]
                errors.extend(e.error_list)  # <----

        # …

See

• ​https://github.com/django/django/blob/2.0.4/django/db/models/fields/__init__.py#L553-L567
• ​https://github.com/django/django/blob/c3055242c81812278ebdc93dd109f30d2cbd1610/django/db/models/fields/__init__.py#L577-L591 (current master)

This leads to an AttributeError when using a dict as message when raising a ValidationError inside a validator.

[Tim Graham]

Could you elaborate on the use case for using a dictionary message inside a validator's ValidationError? I would expect validators to be generic and not know the field names they are being used with.

[Michael Käufl]

Sure. Consider a JSONField with an allowed list of keys:

if key not in allowed_keys:
    raise ValidationError({key: 'Not allowed.'})

We use these exceptions not only for the admin, but also for API error messages through django rest framework's ​custom exception handling. This is also the reason, why we use the field's name in some other (therefore non-generic) validators. Using the field's name as key in the error_dict, allows our front-end to display the error message at the affected field and not as a generic error message of the form.

[Simon Charette]

I think that Field.run_validators() should not to handle dict based ValidationError as they should only be used to map concrete fields to errors in multi-fields cleaning functions (e.g. Form.clean(), Model.clean()). Since Field instances cannot have nested-fields I wouldn't expect dict to be handled.

In your JSONField subclass example you're using an invalid key as your field mapping. This key doesn't map to any field and raising ValidationError(f'Key {key} not allowed.') within the field would be more appropriate.

Keep in mind that errors have to be ultimately flattened to a {field_name -> error_list} map at the form/model level while a different model is used for DRF serializers since they have nested fields.