id summary reporter owner description type status component version severity resolution keywords cc stage has_patch needs_docs needs_tests needs_better_patch easy ui_ux 35909 Mutation of FormMixin.initial can cause a FormView with a formset to crash David Sanders "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 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 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 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 }}}" Bug new Generic views 5.1 Normal FormView FormSet Unreviewed 0 0 0 0 0 0