python - Dictionary comprehension from a list of tuples with lists in them -

suppose have variable structured so:

results = [([123], 456),  ([1, 2, 3], 456),  ([2], 456)] 

i want dictionary using dictionary comprehension achieve doing so:

{item[1]:set(i item in results in item[0]) item in results}  {456: {1, 2, 3, 123}} 

however suppose variable results becomes this:

[([123], 456),  ([1, 2, 3], 456),  ([2], 456),  ([789], 'fizz')] 

i have unsuccessfully attempted change comprehension work on case. how achieve result through use of dictionary comprehension? know how using loops wish learn how achieved using comprehension. thanks!

edit:

output so:

{456: {1, 2, 3, 123}, 'fizz': {789}} 

you can without comprehension, if insist on comprehension, here's solution uses itertools.groupby:

from itertools import groupby import operator  f = operator.itemgetter(1) r = {k: set(i x in g in x[0]) k, g in groupby(results, f)} print(r) # {456: set([3, 1, 2, 123]), 'fizz': set([789])} 

caveat: note no presorting done, since items arranged according group key.

the solution more readable (say more pythonic) simple loop uses defaultdict:

from collections import defaultdict  r = defaultdict(set) v, k in results:    r[k].update(v) print(r) # defaultdict(<type 'set'>, {456: set([3, 1, 2, 123]), 'fizz': set([789])})