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

Paul Moore p.f.moore at gmail.com
Fri Apr 3 14:22:02 CEST 2009

2009/4/3 Hrvoje Niksic <hrvoje.niksic at avl.com>:

I've stumbled upon an oddity using sets.  It's trivial to test if a value is in the set, but it appears to be impossible to retrieve a stored value, other than by iterating over the whole set.  Let me describe a concrete use case.

Imagine a set of objects identified by some piece of information, such as a "key" slot (guaranteed to be constant for any particular element).  The object could look like this: 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)  # ... Now imagine a set "s" of such objects.  I can add them to the set:

s = set() s.add(Element('foo')) s.add(Element('bar')) I can test membership using the keys: 'foo' in s True 'blah' in s False But I can't seem to find a way to retrieve the element corresponding to 'foo', at least not without iterating over the entire set.  Is this an oversight or an intentional feature?  Or am I just missing an obvious way to do this?

My instinct is that it's intentional. I'd say that you're abusing eq here. If you can say "x in s" and then can't use x as if it were the actual item inserted into s, then are they really "equal"?

Using a dict seems like the correct answer. I certainly don't think it's worth complicating the set interface to cover this corner case.

Paul.

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