Append a copy of a model's data to self in __init__

models.py

from django.db import models
from itertools import chain

class ModelName(models.Model):
    def __init__(self, *args, **kwargs):
        super(ModelName, self).__init__(*args, **kwargs)
        if self.pk is not None:
            field_names = [f.name for f in chain(self._meta.fields, self._meta.many_to_many)]
            self._old = dict((fname, getattr(self, fname, None)) for fname in field_names)
        else:
            self._old = {}