How to make a Django Model form Readonly?

pseudo-code (not tested):

class ReadOnlyFormMixin(ModelForm):
    def __init__(self, *args, **kwargs):
        super(ReadOnlyFormMixin, self).__init__(*args, **kwargs)
        for key in self.fields.keys():
            self.fields[key].widget.attrs['readonly'] = True

    def save(self, *args, **kwargs):
        # do not do anything
        pass

class SampleReadOnlyForm(ReadOnlyFormMixin, SampleForm):
    pass