Monkey-patch a Django Field instance in order to automatically add a CSS class to its widget when validation fails

auto_error_class.py

import types

from django.core import exceptions

def auto_error_class(field, error_class="error"):

    inner_clean = field.clean

    def wrap_clean(self, *args, **kwargs):
        try:
            return inner_clean(*args, **kwargs)
        except exceptions.ValidationError as ex:
            self.widget.attrs["class"] = self.widget.attrs.get(
                "class", ""
            ) + " " + error_class
            raise ex

    field.clean = types.MethodType(wrap_clean, field, field.__class__)

    return field