Readonly fields in django formset

I'd recommend specifying a form to use for the model, and in that form you can set whatever attributes you want to read only.

#forms.py
class AuthorForm(forms.ModelForm):
    class Meta:
        model = Author

    def __init__(self, *args, **kwargs):
        super(AuthorForm, self).__init__(*args, **kwargs)
        if self.instance.id:
            self.fields['weight'].widget.attrs['readonly'] = True

#views.py
AuthorFormSet = modelformset_factory(Author, extra=2, form=AuthorForm)