__hash__ is not inherited from model.Model if __eq__ is defined

[Adam Janik]

In my model, that inherits directly from models.Model, I defined my own __eq__:

    class Meta:
        ordering = ('pk', )

    def __eq__(self, other):
        return self.as_data() == other.as_data()

What was strange, when updating to 2.2, classes like above (and instances) do not have __hash__ defined anymore.

>>> from myapp.account.models import Address
>>> Address.__hash__ is None
True
>>> import django
>>> django.VERSION
(2, 2, 0, 'final', 0)

while it was certainly present in 2.1

>>> from myapp.account.models import Address
>>> Address.__hash__
<function Model.__hash__ at 0x7f013aee4d08>
>>> import django
>>> django.VERSION
(2, 1, 8, 'final', 0)

I am not sure if it is intended, as I did not find (or maybe missed) mentions about it in 2.2 release notes.

[Mariusz Felisiak]

Reproduced at 755673e1bca7edb6bee7a958f40d9ae54d85d44c.

Regression in a68ea231012434b522ce45c513d84add516afa60.

[Carlton Gibson]

Ultimately this is a slightly different version of #30254, which was raised after a68ea231012434b522ce45c513d84add516afa60.
(Will resolve as a duplicate.)

Prior to the changes in a68ea231012434b522ce45c513d84add516afa60 the model would
inherit both __eq__() and __hash__() from models.Model during __new__().
(With __eq__() then being replaced in the add_to_class() call later.)

This was incorrect. The Python docs are quite specific:

A class that overrides __eq__() and does not define __hash__() will have its __hash__() implicitly set to None. When the __hash__() method of a class is None, instances of the class will raise an appropriate TypeError when a program attempts to retrieve their hash value, and will also be correctly identified as unhashable when checking isinstance(obj, collections.abc.Hashable).

If a class that overrides __eq__() needs to retain the implementation of __hash__() from a parent class, the interpreter must be told this explicitly by setting __hash__ = <ParentClass>.__hash__.

By passing the defined __eq__() (as well as __hash__() if defined) into __new__(), the change is bringing Django's behaviour into line with Python's.

The changes here were a bug fix, rather than a breaking change per se. (Yes, it is a
change in behaviour, but so are all bugfixes, and we don't list them as
breaking changes in the release notes.)

I've ​added a couple test cases demonstrating the expected behaviour in a PR. In particular the test_missing_hash_not_inherited() cases fails on Django 2.1.

The fix for your project is to explicitly declare the __hash__ attribute as per the docs above:

    __hash__ = models.Model.__hash__

I hope that helps.