inverting a dictionary of lists

[Gabriel Genellina]

En Fri, 15 Jun 2007 00:20:33 -0300, <bpowah at gmail.com> escribió:

> I searched for an hour and don't see a solution to this (i assume
> somewhat common) problem.
>
> I have a very large dictionary of lists:
> d = {a:[1,2], b:[2,3], c:[3]}
> and i want to reverse the associativity of the integers thusly:
> inverse(d)   makes   {1:[a], 2:[a,b], 3:[b,c]}
>
> my solution expands the original dict into two lists of keys and list
> elements:
> list1:  [a,a,b,b,c]
> list2:  [1,2,2,3,3]
> then recombines them with the reverse operation.
>
> but this takes too much time and a lot of memory.
> I wonder if anyone can point me to a more efficient solution?

py> d = dict(a=[1,2], b=[2,3], c=[3])
py> result = {}
py> for k,v in d.iteritems():
...   for item in v:
...     result.setdefault(item, []).append(k)
...
py> result
{1: ['a'], 2: ['a', 'b'], 3: ['c', 'b']}
py>

You may use collections.defaultdict too - search some recent posts.

-- 
Gabriel Genellina