[Python-Dev] Documentation Error for __hash__

[Matt Giuca]

> This may have been true for old style classes, but as new style classes
> inherit a default __hash__ from object - mutable objects *will* be usable as
> dictionary keys (hashed on identity) *unless* they implement a __hash__
> method that raises a type error.
>

I always thought this was a bug in new-style classes (due to the fact that,
as you say, they inherit __hash__ from object whether it's wanted or not).
However, it seems to be fixed in Python 3.0. So this documentation is only
"wrong" for Python 2.x branch.

See:

Python 2.6b3+ (trunk:66055, Aug 29 2008, 07:50:39)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class X(object):
...     def __eq__(self, other):
...             return True
...
>>> x = X()
>>> hash(x)
-1211564180

versus

Python 3.0b3+ (py3k:66055M, Aug 29 2008, 07:52:23)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class X(object):
...     def __eq__(self, other):
...             return True
...
>>> x = X()
>>> hash(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'X'

Matt
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-dev/attachments/20080829/bec02062/attachment.htm>