#29158: ModelChoiceField crashes when checking choices' length
-------------------------------------+-------------------------------------
Reporter: François | Owner: François Freitag
Freitag |
Type: | Status: assigned
Uncategorized |
Component: Forms | Version: 1.8
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
ModelChoiceField must accept Manager as a `.queryset` for backward
compatibility reasons (see #25683). However, `ModelChoiceIterator` does
not play nice with Managers:
{{{
class TestModelChoiceField(TestCase):
def test_queryset_manager_has_length(self):
f = ModelChoiceField(ChoiceOptionModel.objects)
len(f.choices)
}}}
Errors with:
{{{
======================================================================
ERROR: test_queryset_manager_has_length
(forms_tests.test_tmp.TestModelChoiceField)
----------------------------------------------------------------------
Traceback (most recent call last):
File "django/tests/forms_tests/test_tmp.py", line 10, in
test_queryset_manager_has_length
len(f.choices)
File "django/django/forms/models.py", line 1141, in __len__
return len(self.queryset) + (1 if self.field.empty_label is not None
else 0)
TypeError: object of type 'Manager' has no len()
}}}
----
The reason why this `TypeError` was not reported earlier is probably
because Python swallows `TypeError` silently for `__len__` when `list` is
called, because generators have no `len`:
{{{
Python 3.6.4 (default, Jan 5 2018, 02:35:40)
[GCC 7.2.1 20171224] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def gene():
... yield 1
...
>>> len(gene())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'generator' has no len()
>>> list(gene())
[1]
--
Ticket URL: <https://code.djangoproject.com/ticket/29158>
Django <https://code.djangoproject.com/>
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