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

Hrvoje Niksic hrvoje.niksic at avl.com
Fri Apr 3 14:07:02 CEST 2009

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?

I know I can work around this by changing the set of elements to a dict that maps key -> element, but this feels unsatisfactory. It's redundant, as the element already contains all the necessary information, and the set already knows how to use it, and the set must remember the original elements anyway, to be able to iterate over them, so why not allow one to retrieve them? Secondly, the data structure I need conceptually is a set of elements, so it feels wrong to pigeonhole it into a dict.

This wasn't an isolated case, we stumbled on this several times while trying to use sets. In comparison, STL sets don't have this limitation.

If this is not possible, I would like to propose either that set's getitem translates key to value, so that s['foo'] would return the first element, or, if this is considered ugly, an equivalent method, such as s.get('foo').

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