Combing Two Mixins

gistfile1.py

class CityAndStateInputMixin(object):
    def __init__(self, *args, **kwargs):
        super(CityAndStateInputMixin, self).__init__(*args, **kwargs)

        #add custom error_class
        self.error_class = CustomListError

        #adding State Selct Widget and dynamic css edits
        self.fields['state'] = forms.ChoiceField(choices=US_STATES,
                                                 widget=StateSelectWidget)
        self.fields['state'].widget.attrs = {'class': 'custom state',
                                             'placeholder': 'State'}

        #adding custom_city
        self.fields['city'].widget.attrs.update({'class': 'city'})

class AddPlaceHolderMixin(object):
    def __init__(self, *args, **kwargs):
        super(AddPlaceHolderMixin, self).__init__(*args, **kwargs)
        self.add_placeholder_class()

    def add_placeholder_class(self):
        for key, field in self.fields.items():
            if key in self.required_fields:
                try:
                    field.widget.attrs.update(
                        {'placeholder': '{0} (required)'.format(field.label)})
                except KeyError:
                    pass
            else:
                try:
                    field.widget.attrs.update(
                        {'placeholder': '{0}'.format(field.label)})
                except KeyError:
                        pass
                    

class MyCustomForm(CityAndStateInputMixin, AddPlaceHolderMixin):
    def __init__(self, *args, **kwargs):
        return super(MyCustomForm, self).__init__(*args, **kwargs)