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Date: Thu, 19 May 2011 23:44:59 -0700
Subject: Re: hash values and equality
From: Chris Rebert <clp2@rebertia.com>
To: Ethan Furman <ethan@stoneleaf.us>
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On Thu, May 19, 2011 at 10:43 PM, Ethan Furman <ethan@stoneleaf.us> wrote:
> Several folk have said that objects that compare equal must hash equal, a=
nd
> 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? =C2=A0Here's a toy example of a class I'm thinking of wri=
ting
> that will compare equal with int's, but hash differently:
>
> --> class Wierd():
> ... =C2=A0 =C2=A0 def __init__(self, value):
> ... =C2=A0 =C2=A0 =C2=A0 =C2=A0 self.value =3D value
> ... =C2=A0 =C2=A0 def __eq__(self, other):
> ... =C2=A0 =C2=A0 =C2=A0 =C2=A0 return self.value =3D=3D other
> ... =C2=A0 =C2=A0 def __hash__(self):
> ... =C2=A0 =C2=A0 =C2=A0 =C2=A0 return hash((self.value + 13) ** 3)
> ...
> --> one =3D Wierd(1)
> --> two =3D Wierd(2)
> --> three =3D Wierd(3)
> --> one
> <Wierd object at 0x00BFE710>
> --> one =3D=3D 1
> True
> --> one =3D=3D 2
> False
> --> two =3D=3D 2
> True
> --> three =3D=3D 3
> True
> --> d =3D dict()
> --> d[one] =3D '1'
> --> d[two] =3D '2'
> --> d[three] =3D '3'
> --> d
> {<Wierd object at 0x00BFE710>: '1',
> =C2=A0<Wierd object at 0x00BFE870>: '3',
> =C2=A0<Wierd object at 0x00BFE830>: '2'}
> --> d[1] =3D '1.0'
> --> d[2] =3D '2.0'
> --> d[3] =3D '3.0'

This is the part considered "horrible":
> --> d
> {<Wierd object at 0x00BFE870>: '3',
> =C2=A01: '1.0',
> =C2=A02: '2.0',
> =C2=A03: '3.0',
> =C2=A0<Wierd object at 0x00BFE830>: '2',
> =C2=A0<Wierd object at 0x00BFE710>: '1'}

Compare:
>>> x =3D {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 =3D n
...     def __eq__(self, other):
...         return self.n =3D=3D other.n
...
>>> Naughty(5) =3D=3D Naughty(5)
True
>>> Naughty(5) is Naughty(5)
False
>>> bad =3D Naughty(3)
>>> y =3D {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