Raising ValidationError from Serializer.to_internal_value() with error message(s) as list causes ValueError

[ryochiji]

>>> from rest_framework.serializers import Serializer
>>> from rest_framework.exceptions import ValidationError
>>> class FooSerializer(Serializer):
...   def to_internal_value(self, data):
...     raise ValidationError(['foo error'])
... 
>>> foo = FooSerializer(data={})
>>> foo.is_valid(raise_exception=True)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/altschool/vishnu-backend/lib/python2.7/site-packages/rest_framework/serializers.py", line 221, in is_valid
    raise ValidationError(self.errors)
  File "/usr/local/altschool/vishnu-backend/lib/python2.7/site-packages/rest_framework/serializers.py", line 509, in errors
    return ReturnDict(ret, serializer=self)
  File "/usr/local/altschool/vishnu-backend/lib/python2.7/site-packages/rest_framework/utils/serializer_helpers.py", line 20, in __init__
    super(ReturnDict, self).__init__(*args, **kwargs)
  File "/Users/ryochiji/.pyenv/versions/2.7.8/lib/python2.7/collections.py", line 52, in __init__
    self.__update(*args, **kwds)
  File "/usr/local/altschool/vishnu-backend/lib/python2.7/_abcoll.py", line 566, in update
    for key, value in other:
ValueError: too many values to unpack

This seems to happen because Serializer.errors passes self._errors to ReturnDict without type-checking the way ListSerializer.errors does.

[laz2]

Breaking changes in usage self.errors instead self._errors in older versions.

From official documentation:

To create a read-write serializer we first need to implement a .to_internal_value() method. This method returns the validated values that will be used to construct the object instance, and may raise a ValidationError if the supplied data is in an incorrect format.

I think, line value = self.to_internal_value(data) should be under try-except to proper exception converting.

[laz2]

Original to_internal_value always raises exception in dict-format:
raise ValidationError({ api_settings.NON_FIELD_ERRORS_KEY: [message] })

[maryokhin]

This should probably at least be mentioned in the docs somewhere?

[mikeengland]

I'm also experiencing this issue. Raising a validation error inside to_internal_value throws the same exception as above. This can be handled outside to_internal_value so it isn't a blocker by any means.

[practual]

This issue can also be seen if you pass data=None to a serializer, e.g.:

class MySerializer(serializers.Serializer):
    name = serializers.CharField()

    class Meta:
        fields = ('name',)

serializer = MySerializer(data=None)
serializer.is_valid()
serializer.errors
>>> ValueError: too many values to unpack (expected 2)

I believe this error could be avoided by wrapping self.validate_empty_values(data) with a check for self.parent to determine whether the serializer is nested or not. If the serializer is not nested, it should raise the error as a non-field error.

[carltongibson]

I'm going to de-milestone for now. We can reassess after v3.7

[rpkilby]

Hi all, I've opened #5466 with a docs clarification. In short, the recommended approach is:

• Override .validate() when you want to perform object-level validation. You can raise ValidationError('msg') and ValidationError(['msg 1', 'msg 2']) as expected.
• Override .to_internal_value() when you need to alter how data is deserialized. If deserialization fails, it's expected that you raise a ValidationError with a dictionary mapping field names / settings.NON_FIELD_ERRORS_KEY to a list of error messages.

[tomchristie]

#5466 Is a good start, but I think we want to restructure that slightly.

The opening paragraphs for that section might be the right place make your validate vs to_internal_value point, tho I'm open to discussion here.