[Python-ideas] Dict with inverse form

[Mahmoud Hashemi]

You hit the nail on the head that inverting dictionaries has a big problem
with collisions, as values become keys. One solution to this is to use a
MultiDict, a la boltons' OrderedMultiDict:
https://boltons.readthedocs.org/en/latest/dictutils.html#boltons.dictutils.OrderedMultiDict.inverted

But yes, I use inversions all the time, and would love to see a MultiDict
in the collections module to support that use case.

Mahmoud
https://github.com/mahmoud
https://twitter.com/mhashemi
http://sedimental.org

On Thu, Feb 11, 2016 at 6:14 PM, João Bernardo <jbvsmo at gmail.com> wrote:

> Many times I have the need for a dictionary and its inverse form (keys and
> values swapped). For static dictionaries, it is easy, but sometimes I wish
> there were a class to hold and update both dictionaries at the same time.
> Also it will avoid creating 2 variables.
>
> Sample:
>
> # Current code
> d = {'foo': 10, 'bar': 12, 'baz': 17}
> d_inv = {v: k for k, v in d.items()}
>
> # My proposal
> d = InverseDict({'foo': 10, 'bar': 12, 'baz': 17})
>
> d['foo'] == 10
> d.inv[10] == 'foo'
>
> This class could exist in the collections module.
> The way the inverse dictionary is accessed could be different (another
> attribute name or another form):
>
> d.I[10]       # Like numpy's matrix.T for transpose
> d.inverse[10] # More verbose
>
> d[10:1]       # d[10:0] and d[10] would mean the direct dict
>               # while d[10:1] the inverse one.
>               # This would limit the usage to get/set/del only...
>
> (-d)[10]      # negative symbol but it would require parenthesis.
>               # (~d)[10] should be more correct because of
>               # the operator's name
>
> The inverse version of the dict may or may not allow repeated items
> (overwrite vs error). The important part would be to remove keys from the
> inverse dict when the value is changed in the direct one:
>
> d['foo'] = 1
> d.inv[1] == 'foo'
> d.inv[10] -> will raise KeyError instead of returning 'foo'
>
>
> Regards,
> João Bernardo
>
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