[dictionary] how to get key by item

[Skip Montanaro]

    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.

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