[Python-Dev] Getting values stored inside sets (original) (raw)

Raymond Hettinger python at rcn.com
Sat Apr 4 04:37:38 CEST 2009

[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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