[Python-Dev] Getting values stored inside sets
tjreedy at udel.edu
Fri Apr 3 14:26:02 CEST 2009
Hrvoje Niksic wrote:
> 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,
Set elements, by definition, do not have keys or position by which to
grab. When they do, use a dict or list.
> 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
> >>> 'blah' in s
> 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?
Use a dict, like you did.
> I know I can work around this by changing the set of elements to a dict
> that maps key -> element, but this feels unsatisfactory.
Sorry, that is the right way.
> redundant, as the element already contains all the necessary
Records in a database have all the information of the record, but we
still put out fields for indexes.
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