hashkey/digest for a complex object

Diez B. Roggisch deets at web.de
Wed Oct 6 21:26:21 CEST 2010


kj <no.email at please.post> writes:

> The short version of this question is: where can I find the algorithm
> used by the tuple class's __hash__ method?

Surprisingly, in the source:

http://google.com/codesearch/p?hl=de#-2BKs-LW4I0/trunk/python/src/Objects/tupleobject.c&q=python%20tuplehash&sa=N&cd=1&ct=rc

> Now, for the long version of this question, I'm working with some
> complext Python objects that I want to be able to compare for
> equality easily.
>
> These objects are non-mutable once they are created, so I would
> like to use a two-step comparison for equality, based on the
> assumption that I can compute (either at creation time, or as needed
> and memoized) a hashkey/digest for each object.  The test for
> equality of two of these objects would first compare their hashkeys.
> If they are different, the two objects are declared different; if
> they match, then a more stringent test for equality is performed.
>
> So the problem is to come up with a reasonable hashkey for each of
> these objects.  The objects have two significant attributes, and
> two of these objects should be regarded as equal if these attributes
> are "the same" in both.  The first attribute is a simple dictionary
> whose keys are integers and values are strings.  The second attribute
> is more complicated.  It is a tree structure, represented as a
> dictionary of dictionaries of dictionaries... until we get to the
> leaf elements, which are frozensets of strings.  The keys at every
> level of this structure are strings.  E.g. a simple example of such
> an attribute would look like:
>
> {'A': {'a': set(['1', '2', '3']),
>        'b': set(['4', '5'])},
>  'B': set(['6', '7', '8'])}
>
> I'm looking for a good algorithm for computing a hash key for
> something like this?  (Basically I'm looking for a good way to
> combine hashkeys.)

Creating tuples from dicts, recursively, and stabilized by using sorted
on items.

Diez



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