hashkey/digest for a complex object

Steven D'Aprano steve at REMOVE-THIS-cybersource.com.au
Sun Oct 10 02:31:47 CEST 2010

On Sat, 09 Oct 2010 21:39:51 +0100, Arnaud Delobelle wrote:

> 1. hash() is an idempotent function, i.e. hash(hash(x)) == hash(x) hold
> for any hashable x (this is a simple consequence of the fact that
> hash(x) == x for any int x (by 'int' I mean 2.X int)).

It's a beautiful theory, but, alas, it is not the case.

>>> hash(-1) == -1
>>> hash(2**64) == 2**64

to give only two of an infinite number of counter-examples.

Aside: what do you mean by '2.x int'? Do you mean an int in 2.x versions 
before, or after, ints and longs were partially integrated?

[steve at sylar ~]$ python2.1
Python 2.1.3 (#1, Aug 12 2010, 01:53:57)
[GCC 4.1.2 20070925 (Red Hat 4.1.2-27)] on linux2
Type "copyright", "credits" or "license" for more information.
>>> 2**64
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
OverflowError: integer exponentiation

People keep forgetting that 2.2 introduced nearly as many far-reaching 
changes as 3.0.


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