Feature Requests item #1483384, was opened at 2006-05-07 10:41
Message generated for change (Comment added) made by rhettinger
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Category: Extension Modules
Group: None
>Status: Closed
>Resolution: Rejected
Priority: 5
Submitted By: Michael Tsai (michaeltsai)
Assigned to: Raymond Hettinger (rhettinger)
Summary: Add set.member() method
Initial Comment:
Right now, when I check membership in a set, the __in__ method just
returns True/False if there is an object in the set that's == to the
argument. I would like to have a member() method that returns the object
in the set or raises KeyError if the argument is not in the set. This would
be useful for interning and other cases where right now I'd use a
degenerate dictionary where the keys and values are equal.
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>Comment By: Raymond Hettinger (rhettinger)
Date: 2006-05-08 23:36
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The use case is interesting and points-out that the set
API provides no way to retrieve the exact set member that
matches an equality comparison; however, that is
essentially a description of a mapping operation, not a
set operation. The x=d.setdefault(x,x) approach
satisfactorily fulfills the use case (though with a bit
more memory consumption for the value that duplicates the
key).
Also, I'm disinclined to introduce such a method to the
set API which is made simple by a uniform focus on
equality comparisons and intuitive operations. Adding an
identity focused method makes the whole API harder to
understand and less of a power tool suitable for
beginners. IOW, I expect that the method would harm more
than it would help.
I'm rejecting the feature request but want to thank you
for the thought provoking idea.
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Comment By: Michael Tsai (michaeltsai)
Date: 2006-05-08 07:07
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Well, the example you wrote doesn't capture the pattern I was thinking of. I'm
thinking of cases like:
d = {}
#...
x = d.setDefault(x, x)
where I want to get the canonical x. This might be x itself, or it might be
another object that's ==. I doubt you've seen set code that does this with
sets, because I don't think it's possible with the existing set API to do this
efficiently.
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Comment By: Raymond Hettinger (rhettinger)
Date: 2006-05-08 00:37
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I'm curious to see some of your dictionary examples that
do not seem to tranlate cleanly with the existing set API.
In published code, I've not seen people writing anything
like what is being requested, i.e. I haven't seen
fragments like:
if x in s:
return x
else:
raise KeyError
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Comment By: Georg Brandl (gbrandl)
Date: 2006-05-07 15:36
Message:
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Moving to Feature Requests. Note that I do not think Raymond
will agree to this.
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