Replace user-input field with enforced value in Django form

I’m subclassing someone else’s form; I need to remove their input field and replace it with a fixed value.

class MyForm(TheirForm):

    def __init__(self, *args, **kwargs):

        super(MyForm, self).__init__(*args, **kwargs)

        # remove unwanted field
        self.fields.pop('fixedfield')

    def clean(self, *args, **kwargs):

        cleaned_data = super(MyForm, self).clean(*args, **kwargs)

        # Restore field with fixed value
        cleaned_data['fixedfield'] = FIXEDVALUE

        return cleaned_data

Simples…