Django Forms with Multiple Inheritance

Sometimes you want to validate only parts of a Django form. A use case for this would be a user profile. A user may have an extensive user profile, but sometimes you only want to validate a subset of the form fields, e.g. the city and the phone number, when providing a profile edit form. On another page you might want to offer a username change. If you try to validate one of those forms using POST data for the provided fields only, the validation will fail because there are other mandatory fields. In that case, Python's support for multiple inheritance may suit you.

Multiple form inheritance

First, define a form for all the form field subsets you might need to validate.

class FormA(forms.Form):
    username = forms.CharField(required=True, label=_(u'Username'))


class FormB(forms.Form):
    city = forms.CharField(required=True, label=_(u'City'))
    phone = forms.IntegerField(required=True, label=_(u'Phone'))

Then create a third form that extends both the first and the second form. You may also add additional fields.

class FormAB(FormA, FormB):
    language = forms.CharField(required=True, label=_(u'Language'))

Display the forms:

Change Username
{{ formA.as_ul }}

Change City/Phone
{{ formB.as_ul }}

Entire Userprofile
{{ formAB.as_ul }}

The result looks correct:

>>> a = FormA({'username': 'danilo'})
>>> a.is_valid()
True
>>> b = FormB({'city': 'Bern', 'phone': '0791112233'})
>>> b.is_valid()
True
>>> c = FormAB({'username': 'danilo', 'city': 'Bern', 'phone': '0791112233'})
>>> c.is_valid()
False
>>> c = FormAB({'username': 'danilo', 'city': 'Bern',
...             'phone': '0791112233', 'language': 'de'})
>>> c.is_valid()
True