Django: How to check if the user left all fields blank (or to initial values)?

You should set required=False for all fields in the form and override the clean method to do custom validation. See the current documentation for more.

Guess I have to answer my own question.

Apparently, there's an undocumented Form function: has_changed()

>>> f = MyForm({})
>>> f.has_changed()
False
>>> f = MyForm({'name': 'test'})
>>> f.has_changed()
True
>>> f = MyForm({'name': 'test'}, initial={'name': 'test'})
>>> f.has_changed()
False

So this would do nicely as the replacement for form_is_blank() (reverted of course).

If you have put required=True in your forms field or in the model blank=False so is_valid() should return False.

To get this functionality work for subset of Forms used in the actual <form> tag you also need to define

class YourForm(forms.ModelForm):
    def full_clean(self):
        if not self.has_changed():
            self._errors = ErrorDict()
            return

        return super(YourForm, self).full_clean()

so when the user is prompted to fix validation errors it doesnt display errors from the forms which you want to validate only if some value isn't blank (or default).