Removing a field from form.fields previously added to _bound_fields_cache has no effect

[Jan Pieter Waagmeester]

The fix for issue "BaseForm.__getitem__() does unneeded work in the happy path" (#32901) added to main in ​https://github.com/django/django/commit/edde2a069929c93e37835dc3f7c9a229040058e2, A shortcut was added to retrieve bound fields from form._bound_fields_cache.

If a form removes a field after the cache entry for that field was created, the field can be still retrieved from the cache, even though the field is not in form.fields anymore:

import django
from django import forms
from django.test import SimpleTestCase

class Form(forms.Form):
    action = forms.CharField()

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        # The form was iterated (in my case, separating basic and advanced fields)
        [field for field in self if getattr(field, 'advanced', False)]

        # After that, we remove a field (i.e. because it is not relevant for admins)
        del self.fields["action"]


class FormTestCase(SimpleTestCase):
    def test_form(self):
        """
        The field should be removed from the instance,
        but it can be still retrieved from self._bound_fields_cache.
        """
        print("django version", django.VERSION)
        form = Form()

        with self.assertRaises(KeyError):
            form["action"]

This succeeds with django==3.2:

pip install django==3.2.16
...
./manage.py test tests
System check identified no issues (0 silenced).
django version (3, 2, 16, 'final', 0)
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

But fails with django==4.0.8 (and django==4.1.3):

pip install django==4.0.8
...
./manage test tests
Found 1 test(s).
System check identified no issues (0 silenced).
django version (4, 0, 8, 'final', 0)
F
======================================================================
FAIL: test_form (test_forms.FormTestCase)
The field should be removed from the instance,
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jieter/workspace/django-test/tests/test_forms.py", line 27, in test_form
    form["action"]
AssertionError: KeyError not raised

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (failures=1)

[Carlton Gibson]

Thanks for the report.

Looks like a bug yes. I think likely we just need to revert edde2a069929c93e37835dc3f7c9a229040058e2. 🤔
(with added regression test for this case.)

[Francesco Panico]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 51faf4bd:

Fixed #34148 -- Reverted "Fixed #32901 -- Optimized BaseForm.getitem()."

This reverts commit edde2a069929c93e37835dc3f7c9a229040058e2.

Thanks Jan Pieter Waagmeester for the report.