hash values and equality

[Chris Rebert]

On Thu, May 19, 2011 at 10:43 PM, Ethan Furman <ethan at stoneleaf.us> wrote:
> Several folk have said that objects that compare equal must hash equal, and
> the docs also state this
> http://docs.python.org/dev/reference/datamodel.html#object.__hash__
>
> I'm hoping somebody can tell me what horrible thing will happen if this
> isn't the case?  Here's a toy example of a class I'm thinking of writing
> that will compare equal with int's, but hash differently:
>
> --> class Wierd():
> ...     def __init__(self, value):
> ...         self.value = value
> ...     def __eq__(self, other):
> ...         return self.value == other
> ...     def __hash__(self):
> ...         return hash((self.value + 13) ** 3)
> ...
> --> one = Wierd(1)
> --> two = Wierd(2)
> --> three = Wierd(3)
> --> one
> <Wierd object at 0x00BFE710>
> --> one == 1
> True
> --> one == 2
> False
> --> two == 2
> True
> --> three == 3
> True
> --> d = dict()
> --> d[one] = '1'
> --> d[two] = '2'
> --> d[three] = '3'
> --> d
> {<Wierd object at 0x00BFE710>: '1',
>  <Wierd object at 0x00BFE870>: '3',
>  <Wierd object at 0x00BFE830>: '2'}
> --> d[1] = '1.0'
> --> d[2] = '2.0'
> --> d[3] = '3.0'

This is the part considered "horrible":
> --> d
> {<Wierd object at 0x00BFE870>: '3',
>  1: '1.0',
>  2: '2.0',
>  3: '3.0',
>  <Wierd object at 0x00BFE830>: '2',
>  <Wierd object at 0x00BFE710>: '1'}

Compare:
>>> x = {5.0 : 'foo'}
>>> x[5]
'foo'

Here's a more common/plausible "horrible" case closer to what the docs
writers had in mind:
>>> class Naughty(object):
...     def __init__(self, n):
...         self.n = n
...     def __eq__(self, other):
...         return self.n == other.n
...
>>> Naughty(5) == Naughty(5)
True
>>> Naughty(5) is Naughty(5)
False
>>> bad = Naughty(3)
>>> y = {bad : 'foo'}
>>> y[bad] # just happens to work
'foo'
>>> del bad
>>> # ok, how do we get to 'foo' now?
>>> y[Naughty(3)] # try the obvious way
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: <__main__.Naughty object at 0x2a1cb0>
>>> # We're screwed.

Naughty instances (and similar) can't be used sensibly as hash keys
(unless you /only/ care about object identity; this is often not the
case).

Cheers,
Chris
--
http://rebertia.com