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Sat Apr 4 04:37:38 CEST 2009 · https://mail.python.org/pipermail/python-dev/2009-April/088190.html

[Nick Coghlan]
> It doesn't quite work the way RDM desribed it - he missed a step.

Thanks for the clarification.  We ought to write-out the process somewhere in a FAQ.

It may also be instructive to step through the recipe that answers the OP's
original request, http://code.activestate.com/recipes/499299/ 

The call "get_equivalent(set([1, 2, 3]), 2.0)" wraps the 2.0 in a new
object t and calls "t in set([1,2,3])".  The set.__contains__ method
hashes t using t.__hash__(self) and checks for an exact match 
using t.__eq__(other).  Both calls delegate to float objects but the 
latter also records the "other" that resulted in a successful equality
test (i.e. 2 is the member of the set that matched the 2.0).  The
get_equivalent call then returns the matching value, 2.0.

As far as I can tell, the technique is completely generic and lets
you reach inside any function or container to retrieve the "other"
value that is equivalent to "self".

Raymond

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[Python-Dev] Getting values stored inside sets