Django Models and the ==, 'in' and 'is' operators

base.py

def __eq__(self, other):
        return isinstance(other, self.__class__) and self._get_pk_val() == other._get_pk_val()

django shell

$ python manage.py shell
Python 2.7.3 (default, Apr 20 2012, 22:44:07) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from project.app.models import Animal
>>> cat = Animal(species="feline")
>>> dog = Animal(species="canine")
>>> cat.species == dog.species
False
>>> cat is dog
False
>>> # A cat is not a dog. Duh.
>>> # However...
>>> cat in [dog]
True
>>> # Oh no! How did the cat get in the dog? Wait a minute...
>>> cat == dog
True
>>> # What?!
>>>

django shell again

>>> # Private keys don't exist.
>>> cat.pk is None
True
>>> dog.pk is None
True
>>> # Saving generates the private key.
>>> cat.save()
>>> cat.pk
2
>>> # == and in operators now work as expected.
>>> cat == dog
False
>>> cat in [dog]
False
>>> 

models.py

from django.db import models

class Animal(models.Model):
    species = models.CharField(max_length=100)

python shell

$ python
Python 2.7.3 (default, Apr 20 2012, 22:44:07) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class Animal():
...     def __init__(self, species):
...         self.species = species
... 
>>> cat = Animal(species="feline")
>>> dog = Animal(species="canine")
>>> cat.species == dog.species
False
>>> cat is dog
False
>>> cat in [dog]
False
>>> cat == dog
False
>>>