How should I collapse elements in Python? [duplicate]

Question

For example:

l = [('a',1),('b',2),('a',2)]

collapsed_l = dict(a=[1,2],b=[2])

How best to get from l to collapsed_l?

In a sense, I want some way of generalising what "field" I'm collapsing, and by which field. I think this is similar to what pivot tables do in databases and spreadsheets, but I may be wrong.

Answer 1

>>> from itertools import groupby
>>> from operator import itemgetter
>>> l = [('a',1),('b',2),('a',2)]
>>> dict((k,[n for l,n in v]) for k,v in groupby(sorted(l),itemgetter(0)))
{'a': [1, 2], 'b': [2]}

Not sure if order of the collapsed values matters, if so you can edit sorted(l) to sorted(l,key=itemgetter(0))