hashkey/digest for a complex object

kj no.email at please.post
Wed Oct 6 20:58:12 CEST 2010

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

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.)



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