[dictionary] how to get key by item
Keith Dart
kdart at kdart.com
Tue Dec 14 02:15:24 EST 2004
Skip Montanaro wrote:
> Egor> i know how to get item by key
> ...
> Egor> but i wonder how to get key by item
>
> Assuming your dictionary defines a one-to-one mapping, just invert it:
>
> >>> forward = {10 : 50, 2 : 12, 4 : 43}
> >>> reverse = dict([(v,k) for (k,v) in forward.iteritems()])
> >>> print forward
> {10: 50, 4: 43, 2: 12}
> >>> print reverse
> {50: 10, 43: 4, 12: 2}
>
> That doubles your storage, so you'll have to trade that off against the
> speed gain of not having to loop over the entire dictionary.
>
> Skip
But beware that all the items in the original dictionary must be
hashable. The example shows just integers, so I assume they are in this
case. But generally, this may not work.
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