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
24706	ModelForm._post_clean assumes that setting attributes onto an instance cannot raise ValidationError	Keryn Knight	Keryn Knight	"Currently, [https://github.com/django/django/blob/master/django/forms/models.py#L433 ModelForm._post_clean] calls out to `construct_instance` which, under the hood, uses `setattr(instance, self.name, data)`, assuming that no cleverness is taking place, so doesn't attempt to catch `ValidationError` as subsequent lines do (when calling `full_clean` and `validate_unique`)

This precludes doing things like using `__setattr__` on the model, or replacing raw attribute access with descriptors (which is [https://github.com/kezabelle/django-strictmodels/blob/master/strictmodels.py#L14 what I did] to encounter this) that do validation checks on assignment rather than remembering to call `full_clean` everywhere.

As far as I can see, this is the only part of the `_post_clean` implementation where ValidationError could conceivably be being thrown which isn't already handled.

Without catching the exception, it I think necessitates using a custom ModelForm subclass just to do something like:
{{{
class MySubForm(ModelForm):
    def _post_clean(self):
        try:
            super(MySubForm, self)._post_clean()
        except ValidationError as e:
            self._update_errors(e)
}}}
to avoid the following stacktrace:
{{{
django/forms/forms.py:184: in is_valid
    return self.is_bound and not self.errors
django/forms/forms.py:176: in errors
    self.full_clean()
django/forms/forms.py:394: in full_clean
    self._post_clean()
django/forms/models.py:427: in _post_clean
    self.instance = construct_instance(self, self.instance, opts.fields, construct_instance_exclude)
django/forms/models.py:62: in construct_instance
    f.save_form_data(instance, cleaned_data[f.name])
django/db/models/fields/__init__.py:874: in save_form_data
    setattr(instance, self.name, data)
strictmodels.py:42: in __set__
    new_value = self.field.clean(value=value, model_instance=instance)
django/db/models/fields/__init__.py:589: in clean
    self.run_validators(value)

ValidationError: ['error string', 'etc']
}}}

As you can see, the exception is the result of '''my code''', but has broken the ''implied'' contract of `.is_valid()` being that it suppresses and collects `ValidationErrors`."	Cleanup/optimization	closed	Forms	dev	Normal	fixed	1.9	django@…	Ready for checkin	1	0	0	0	0	0