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

Paul Moore p.f.moore at gmail.com
Fri Apr 3 18:57:29 CEST 2009

2009/4/3 Steven D'Aprano <steve at pearwood.info>:

Python does not promise that if x == y, you can use y anywhere you can use x. Nor should it. Paul's declaration of abuse of eq is unfounded.

Sorry, I was trying to simplify what I was saying, and simplified it to the point where it didn't make sense :-) Martin (quoted below) explained what I was trying to say far more clearly.

2009/4/3 "Martin v. Löwis" <martin at v.loewis.de>:

If you have a set of elements, and you check "'foo' in s", then you should be able just to use the string 'foo' itself for whatever you want to do with it - you have essentially created a set of strings. If you think that 'foo' and Element('foo') are different things, you should not implement eq in a way that they are considered equal.

-- in particular, if you're using things in sets (which are all about equality, insofar as that's how "duplicates" are defined) you should ensure that your definition of eq respects the idea that equal objects are duplicates (ie, interchangeable). Otherwise, a dict is the appropriate data structure.

Actually, given the definition in the original post,

class Element(object): def init(self, key): self.key = key def eq(self, other): return self.key == other def hash(self): return hash(self.key)

as far as I can tell, equality is only defined between Elements and keys - not even between 2 elements! So with that definition, there could be many Elements in a set, all equal to the same key. Which is completely insane.

In fact, Python seems to be doing something I don't understand:

class Element(object): ... def init(self, key, id): ... self.key = key ... self.id = id ... def eq(self, other): ... print "Calling eq for %s" % self.id ... return self.key == other ... def hash(self): ... return hash(self.key) ... a = Element('k', 'a') b = Element('k', 'b') a == b Calling eq for a Calling eq for b True a == a Calling eq for a Calling eq for a True

Why does eq get called twice in these cases? Why does a == b, as that means a.key == b, and clearly a.key ('k') does not equal b. Or are there some further options being tried, in str,eq or object.eq? The documentation doesn't say so... Specifically, there's nothing saying that a "reversed" version is tried.

Paul.

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