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List of tuples from (a, all b) to (b, all a)

Tag: python,list,python-2.7,tuples

I am starting with a list of tuples (a,all b). I want to end with a list of tuples (b,all a).

For example:

FROM  
(a1,[b1,b2,b3])  
(a2,[b2])  
(a3,[b1,b2])

TO  
(b1,[a1,a3])  
(b2[a1,a2,a3])  
(b3,[a1]

How do I do this using Python 2? Thank you for your help.

Best How To :

You can use collections.defaultdict:

tups = [
    ('a1',['b1','b2','b3']),
    ('a2',['b2']),
    ('a3',['b1','b2'])
]

d = collections.defaultdict(list)
for a, bs in tups:
    for b in bs:
        d[b].append(a)

Then:

>>> d.items()
[('b1', ['a1', 'a3']), ('b2', ['a1', 'a2', 'a3']), ('b3', ['a1'])]

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