#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/> The Web framework for perfectionists with deadlines. -- You received this message because you are subscribed to the Google Groups "Django updates" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-updates+unsubscr...@googlegroups.com. To post to this group, send email to django-updates@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/django-updates/058.3003c1605dda325bb31676e58323b746%40djangoproject.com. For more options, visit https://groups.google.com/d/optout.