formset.cleaned_data raises AttributeError when is_valid is True

[Andy Durdin]

With a formset with no initial forms and empty extra forms, formset.is_valid() returns True, but formset.cleaned_data raises an AttributeError.

This can be found with the examples from ​the formsets docs:

>>> from django import forms
>>> class ArticleForm(forms.Form):
...     title = forms.CharField()
...     pub_date = forms.DateField()
... 
>>> from django.forms.formsets import formset_factory
>>> ArticleFormSet = formset_factory(ArticleForm)
>>> formset = ArticleFormSet({})
>>> 
>>> formset.is_valid()
True
>>> formset.errors
[{}]
>>> formset.cleaned_data
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/andy/dev/django/dj/django/forms/formsets.py", line 128, in _get_cleaned_data
    return [form.cleaned_data for form in self.forms]
AttributeError: 'ArticleForm' object has no attribute 'cleaned_data'

[Mark Lavin]

This formset does have a form in it and it is not valid. The formset itself is bound but the form is not bound (and hence not valid).

>>> from django import forms
>>> class ArticleForm(forms.Form):
...     title = forms.CharField()
...     pub_date = forms.DateField()
... 
>>> from django.forms.formsets import formset_factory
>>> ArticleFormSet = formset_factory(ArticleForm)
>>> formset = ArticleFormSet({})
>>> formset.forms
[<__main__.ArticleForm object at 0x94636cc>]
>>> formset.forms[0].is_valid()
False
>>> formset.is_bound
True
>>> formset.forms[0].is_bound
False

I think the problem is in the BaseFormSet._construct_form:

    def _construct_form(self, i, **kwargs):
        """
        Instantiates and returns the i-th form instance in a formset.
        """
        defaults = {'auto_id': self.auto_id, 'prefix': self.add_prefix(i)}
        if self.data or self.files:
            defaults['data'] = self.data
            defaults['files'] = self.files
...

if self.data or self.files: should be if self.is_bound: because the empty data dictionary is not being passed down to the forms.

[Mark Lavin]

Change to BaseFormSet._construct_form with tests

[Mark Lavin]

After coming back to this and speaking with kmtracy I'm not sure the described functionality is correct. A valid formset should have cleaned_data attribute but a formset passed an empty dict should raise a ValidationError "ManagementForm data is missing or has been tampered with". The formset validation needs some work but I believe the docs need an update as well.

[Mark Lavin]

New patch vs [13738] with doc updates

[Mark Lavin]

New patch which updates the validation logic and the docs around formsets. This does introduce a small backwards incompatible change in that previously passing an empty dict would validate but now it will throw and exception that the Management Form is invalid which I believe is the more intuitive behavior. Let me know if you think it needs some more work.

[Karen Tracey]

Removing myself as owner since I'm not going to be able to finish this tonight and then I'll be gone, so if someone else wants to take a look feel free.

[Honza Král]

(In [14667]) Fixed #11418 -- formset.cleaned_data no longer raises AttributeError when is_valid is True. Thanks mlavin!

This also introduces a slightly backwards-incompatible change in
FormSet's behavior, see the release docs for details.