Django Forms add_error() backport for <1.7

add_error.py

from django import forms
from django.forms.forms import NON_FIELD_ERRORS

class AddErrorMixin(object):
    "Backport add_error() for django <1.7"
    def add_error(self, field, msg):
        field = field or NON_FIELD_ERRORS
        if field in self._errors:
            self._errors[field].append(msg)
        else:
            self._errors[field] = self.error_class([msg])

class ExampleForm(AddErrorMixin, forms.Form):
    pass