Returning cleaned_data when overwriting clean() method in Django Model forms

Question

I need to overwrite the clean() method in a Django Model form to perform additional uniqueness checks on the data entered.

This page gives implementation details: https://docs.djangoproject.com/en/1.11/ref/forms/validation/ Copied here:

 def clean(self):
    cleaned_data = super(ContactForm, self).clean()
    cc_myself = cleaned_data.get("cc_myself")
    subject = cleaned_data.get("subject")

    if cc_myself and subject:
        # Only do something if both fields are valid so far.
        if "help" not in subject:
            raise forms.ValidationError(
                "Did not send for 'help' in the subject despite "
                "CC'ing yourself."
            )

However I'm confused why this method doesn't return cleaned_data at the end of the function? Surely that is the correct thing to do?

Answer 1

Have a look at django's _clean_form method:

def _clean_form(self):
    try:
        cleaned_data = self.clean()
    except ValidationError as e:
        self.add_error(None, e)
    else:
        if cleaned_data is not None:
            self.cleaned_data = cleaned_data

Read the last bullet point on the forms doc and especially this bit of the ModelForm doc.

If the clean method raises a ValidationError, the error gets added to the forms' errors. If the clean method has returned anything and threw no errors, the form is going to use that for its cleaned_data attribute. Otherwise it will keep its 'old' one.

In your case, all that your clean method does, is validating an aspect of the form.