django/tests/field_subclassing/fields.py

from django.db import models
from django.db.models.query_utils import DeferredAttribute


class CustomTypedField(models.TextField):
    def db_type(self, connection):
        return 'custom_field'


class CustomDeferredAttribute(DeferredAttribute):
    def __get__(self, instance, cls=None):
        self._count_call(instance, 'get')
        return super().__get__(instance, cls)

    def __set__(self, instance, value):
        self._count_call(instance, 'set')
        instance.__dict__[self.field.attname] = value

    def _count_call(self, instance, get_or_set):
        count_attr = '_%s_%s_count' % (self.field.attname, get_or_set)
        count = getattr(instance, count_attr, 0)
        setattr(instance, count_attr, count + 1)


class CustomDescriptorField(models.CharField):
    descriptor_class = CustomDeferredAttribute