Don't require forms clean_* methods to return a value

[Aymeric Augustin]

Adding custom validation to forms isn't as DRY as it colud be:

    class MyForm(forms.ModelForm):

        class Meta:
            model = MyModel

        def clean_title(self):
            title = self.cleaned_data["title"]
            validate_title(title)  # custom validation
            return title

        def clean_slug(self):
            slug = self.cleaned_data["slug"]
            validate_slug(slug)   # custom validation
            return slug

The requirement that clean() return a cleaned_data dict was recently lifted; what about doing the same for clean_*()? Then the example above could be simplified to:

    class MyForm(forms.ModelForm):

        class Meta:
            model = MyModel

        def clean_title(self):
            validate_title(self.cleaned_data["title"])  # custom validation

        def clean_slug(self):
            validate_slug(self.cleaned_data["slug"])   # custom validation

Otherwise developers go for private APIs, eg.:

    class MyForm(forms.ModelForm):

        class Meta:
            model = MyModel

        def __init__(self, *args, **kwargs):
            super(MyForm, self).__init__(*args, **kwargs)
            self.fields["title"].validators.append(validate_title)
            self.fields["slug"].validators.append(validate_slug)

[Simon Charette]

Maybe it was overlooked to allow clean_*() to return None?

[Marc Tamlyn]

We can't do this, as None is potentially a valid value for the field. It is conceivable for a field to have a non-None value beforehand, and a clean method to modify it to become None. In the case of clean(), this is not a problem as None is not a valid value for self.cleaned_data.

[Aymeric Augustin]

I has a sad, but so be it...