fields.py

from django import forms
from django.db import models

from south.modelsinspector import add_introspection_rules


class ParsleyDecimalFormField(forms.DecimalField):
    def widget_attrs(self, widget):
        attrs = super().widget_attrs(widget)
        # ...
        return attrs


class ParsleyDecimalField(models.DecimalField):
    def formfield(self, **kwargs):
        defaults = {'form_class': ParsleyDecimalFormField}
        defaults.update(kwargs)
        return super().formfield(**defaults)

add_introspection_rules([], ['^\.ParsleyDecimalField'],)
add_introspection_rules([], ['^myapp\.fields\.ParsleyDecimalField'],)