Tracking changes on properties in Django

track_data.py

# Tested against Django 3.2.14.

from django.db.models.base import DEFERRED


def track_data(*fields):
    """Tracks property changes on a model instance.

    Simplified version of David Cramer's gist:
    https://gist.github.com/dcramer/730765

    Please note that this decorator is not meant to be used in mixins
    or generally in any form of class hierarchy. You should also only
    track fields that exist on the class this is added to.

    """

    def inner(cls):
        def _store_current_field_values(self):
            self.__data = {}
            for field in fields:
                try:
                    self.__data[field] = getattr(self, field)
                except KeyError:
                    continue

        cls._store_current_field_values = _store_current_field_values

        def has_changed(self, field):
            return self.__data[field] != getattr(self, field)

        cls.has_changed = has_changed

        def old_value(self, field):
            return self.__data.get(field)

        cls.old_value = old_value

        def new_init(self, *args, **kwargs):
            # If we get deferred fields in here, trying to track their
            # values it will send us into infinite recursion as the
            # model instance is reinitialized for each field being
            # queried from the database. This happens e.g. when deleting
            # a model instance, where tracking is irrelevant, so skip it
            # in that case.
            has_deferred_fields = DEFERRED in args

            new_init._original_init(self, *args, **kwargs)

            if not has_deferred_fields:
                self._store_current_field_values()

        new_init._original_init = cls.__init__
        cls.__init__ = new_init

        def new_save(self, *args, **kwargs):
            new_save._original_save(self, *args, **kwargs)
            self._store_current_field_values()

        new_save._original_save = cls.save
        cls.save = new_save

        return cls

    return inner