TypedChoiceField.has_changed behaviour has changed between 1.8 and 1.9

[Arthur Vuillard]

We've met a strange bug with Django 1.9 on TypedChoiceField.has_changed.

We encountered the bug when using a TypedChoiceField in a form of a formset. When we have an empty extra form, and we submit all the form, the submitted form is not validated due to some "changes" made to the TypedChoiceFields.

The excepted behavior would be to save the formset data without treating that extra form.

I finally found that the method has_changed of a field has been modified from 1.8, and calls self._coerce for the initial value. By removing it, my use case now works like expected.

I attach to this ticket a unit test that reproduces my case.

[Arthur Vuillard]

Unit test

[Tim Graham]

I haven't completely verified the report, but bisected the change to 8714403614c4dfa37e806db4a6708b8a91a827a4.

[Claude Paroz]

It would help to know a bit more about the use case, notably because your test didn't set the coerce argument. Could you provide a little more context, also about the form classes involved?

[Arthur Vuillard]

In fact, in my case, the coerce is used using the field's to_python method as following (my mistake, sorry) :

from django.db import models
from django.forms import TypedChoiceField

CHOICES = [
    ('choice1', 'Choice 1'),
    ('choice2', 'Choice 2'),
    ('choice3', 'Choice 3')
]

my_choices = models.CharField(
    max_length=7,
    choices=CHOICES,
    null=True,
    blank=True,
    default=None,
)


kwargs = {
    'label': 'Method',
    'choices': [
        ('', '---------'),
        ('choice1', 'Choice 1'),
        ('choice2', 'Choice 2'),
        ('choice3', 'Choice 3')
    ],
    'initial': None,
    'empty_value': None,
    'required': False,
    'help_text': '',
    'coerce': my_choices.to_python,
}

tcf = TypedChoiceField(**kwargs)
# the following line did work in django 1.8
assert not tcf.has_changed(None, '')

Overall, I detected that when using my model in a formset having an extra=1. When submitting without touching anything, the form is treated as a changed form due to this changed field (where it should be ignored as nothing was changed).

[Claude Paroz]

That's an issue with CharField and null=True (which is discouraged). However, I think the ​PR fixes this.

[Arthur Vuillard]

I can confirm that the PR fixes the bug on my code. Thanks for that !

[Claude Paroz <claude@…>]

In d91cc25:

Fixed #25942 -- Fixed TypedChoiceField.has_changed with nullable field

This fixes a regression introduced by 871440361.

[Claude Paroz <claude@…>]

In ff077cd:

[1.9.x] Fixed #25942 -- Fixed TypedChoiceField.has_changed with nullable field

This fixes a regression introduced by 871440361.
Backport of d91cc25a2a from master.