Opened 21 hours ago
Closed 20 hours ago
#35909 closed Bug (invalid)
Mutation of FormMixin.initial can cause a FormView with a formset to crash
Reported by: | David Sanders | Owned by: | |
---|---|---|---|
Component: | Generic views | Version: | 5.1 |
Severity: | Normal | Keywords: | FormView FormSet |
Cc: | Triage Stage: | Unreviewed | |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description (last modified by )
Hi Djangonauts, long time no see… have been super busy, hope you're all well!
Here's an interesting problem with some backstory for which I never knew Django behaved like this.
So some of the old timers may know that FormMixin.initial
is a mutable class attribute, and there have been past bug reports concerning this, the main one from 13 years ago is #16138
The fix for this was to call .copy()
during get_initial()
.
However, I just spent a couple of days tracking down and fixing a 500 which was caused by this 😅 The direct cause for this was a junior dev mutating initial
directly, not realising that because it is a class attribute then it's a long-living mutation.
Example
Given this simple example model setup:
class Foo(Model): foo = IntegerField() class Bar(Model): foo = IntegerField()
and these 2 views:
class FooCreateView(CreateView): model = Foo fields = ["foo"] def __init__(self, *args, **kwargs): super().__init__(*args, **kwargs) self.initial["foo"] = 2 class BarCreateView(CreateView): model = Bar fields = ["foo"]
If you first visit FooCreateView
, then visit BarCreateView
; then this will have the unintended side-effect of initialising BarCreateView
with 2
.
This alone doesn't cause a 500 but it is a source of potential bug that may go unnoticed!
Furthermore if you create a FormView
with a model formset, this will cause it to 500!
BarFormSet = modelformset_factory(Bar, fields=["foo"]) class LotsOfBarCreateView(FormView): template_name = "bars.html" form_class = BarFormSet
The unintended initialisation is incompatible with the way initialisation works with formsets - formsets expect a list, not a dictionary - see the traceback below
Potential Solution?
If we simply move the initialisation of initial
to an __init__()
the problem goes away. Furthermore I ran the full test suite with this change and nothing seems to break.
Claude seems to the only active contributor that was part of the original ticket - Would you (or anyone else) know why the solution wasn't to initialise from __init__()
to begin with? I did see mention of it in a duplicate ticket #16701. The original ticket appears to be from prior GitHub times so there's no PR discussion to review.
index ebd071cf00..bff9ac68ce 100644 --- a/django/views/generic/edit.py +++ b/django/views/generic/edit.py @@ -13,11 +13,15 @@ from django.views.generic.detail import ( class FormMixin(ContextMixin): """Provide a way to show and handle a form in a request.""" - initial = {} + initial = None form_class = None success_url = None prefix = None + def __init__(self, *args, **kwargs): + super().__init__(*args, **kwargs) + self.initial = {} + def get_initial(self): """Return the initial data to use for forms on this view.""" return self.initial.copy()
Traceback
Here's the traceback:
Traceback (most recent call last): File "/path/to/test-project/django/core/handlers/exception.py", line 55, in inner response = get_response(request) ^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/core/handlers/base.py", line 220, in _get_response response = response.render() ^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/response.py", line 114, in render self.content = self.rendered_content ^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/response.py", line 92, in rendered_content return template.render(context, self._request) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/backends/django.py", line 107, in render return self.template.render(context) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 171, in render return self._render(context) ^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 163, in _render return self.nodelist.render(context) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 1016, in render return SafeString("".join([node.render_annotated(context) for node in self])) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 1016, in <listcomp> return SafeString("".join([node.render_annotated(context) for node in self])) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 977, in render_annotated return self.render(context) ^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 1081, in render return render_value_in_context(output, context) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 1058, in render_value_in_context value = str(value) ^^^^^^^^^^ File "/path/to/test-project/django/forms/utils.py", line 55, in render return mark_safe(renderer.render(template, context)) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/forms/renderers.py", line 29, in render return template.render(context, request=request).strip() ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/backends/django.py", line 107, in render return self.template.render(context) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 171, in render return self._render(context) ^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 163, in _render return self.nodelist.render(context) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 1016, in render return SafeString("".join([node.render_annotated(context) for node in self])) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 1016, in <listcomp> return SafeString("".join([node.render_annotated(context) for node in self])) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/base.py", line 977, in render_annotated return self.render(context) ^^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/template/defaulttags.py", line 199, in render len_values = len(values) ^^^^^^^^^^^ File "/path/to/test-project/django/forms/formsets.py", line 121, in __len__ return len(self.forms) ^^^^^^^^^^ File "/path/to/test-project/django/utils/functional.py", line 47, in __get__ res = instance.__dict__[self.name] = self.func(instance) ^^^^^^^^^^^^^^^^^^^ File "/path/to/test-project/django/forms/formsets.py", line 205, in forms return [ ^ File "/path/to/test-project/django/forms/formsets.py", line 206, in <listcomp> self._construct_form(i, **self.get_form_kwargs(i)) File "/path/to/test-project/django/forms/models.py", line 740, in _construct_form kwargs["initial"] = self.initial_extra[i - self.initial_form_count()] ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ KeyError: 0
Change History (3)
comment:1 by , 21 hours ago
Description: | modified (diff) |
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comment:2 by , 21 hours ago
Description: | modified (diff) |
---|
comment:3 by , 20 hours ago
Resolution: | → invalid |
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Status: | new → closed |
Hi David, long time no see!
So it looks like we don't have tests covering setting the initial data but I would expect folks to either set the
initial
attribute or overrideget_initial()
, I also feel like that's in the docs although there isn't an explicit example: https://docs.djangoproject.com/en/5.1/ref/class-based-views/mixins-editing/#formmixinSo in short, I think what they have done is not how it should be used and so I wouldn't call this a bug and would close as invalid
I would be open to adding more test coverage (and maybe doc updates to make the above clearer)
Something like (with better naming)...
tests/generic_views/test_edit.py
(self):