MultipleChoiceField validation against a list with a single null element

[Jeff Bauer <jbauer@…>]

MultipleChoiceField fails validation when required=False against a list with a single null value (as opposed to when value itself is null)

>>> f = MultipleChoiceField(required=False, choices=[('1', '1'), ('2', '2')])
>>> f.clean([''])
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "django/newforms/fields.py", line 386, in clean
    if val not in valid_values:
django.newforms.util.ValidationError: [u'Select a valid choice.  is not one of the available choices.']
>>> 

[Adrian Holovaty]

In your example, shouldn't f.clean(['']) return [] rather than ['']?

[Jeff Bauer <jbauer@…>]

replaces 4490.diff patch

[Jeff Bauer <jbauer@…>]

New patch. Clean now returns an empty list rather than the original value.

[Malcolm Tredinnick]

This has already been fixed at some point in the interim (and we're testing it).

[Joshua Smith]

This is still happening for me in Django 3.1.13:

>>> from django.forms import MultipleChoiceField
>>> f = MultipleChoiceField(required=False, choices=[('1', '1'), ('2', '2')])
>>> f.clean([''])
Traceback (most recent call last):
  File "/Users/joshsmith/.pyenv/versions/3.9.4/lib/python3.9/code.py", line 90, in runcode
    exec(code, self.locals)
  File "<console>", line 1, in <module>
  File "/Users/joshsmith/code/course-platform/.venv/lib/python3.9/site-packages/django/forms/fields.py", line 150, in clean
    self.validate(value)
  File "/Users/joshsmith/code/course-platform/.venv/lib/python3.9/site-packages/django/forms/fields.py", line 883, in validate
    raise ValidationError(
django.core.exceptions.ValidationError: ['Select a valid choice.  is not one of the available choices.']

[Carlton Gibson]

So, it's hard to say about the dark-history here, as bootstrapping a very old environment is somewhat fiddly these days but ​the logic here is virtually unchanged from the ​earliest days of the MultipleChoiceField implementation. This behaviour is certainly unchanged as far back as Django 1.8. Without a serious case being made, the backwards compatibility concerns preclude changing this now.

However, for me, this is expected behaviour: '' is not a valid choice. If I want filtering of empty values then I'm expecting to use a widget to filter this in value_from_datadict, or else a field subclass, if I want the behaviour there. The default implementation is doing the right thing here rejecting the incorrect value.