How to group a list of tuples/objects by similar index/attribute in python?

python list grouping
Aufwind picture Aufwind · Jul 6, 2011 · Viewed 32k times · Source

Given a list

old_list = [obj_1, obj_2, obj_3, ...]

I want to create a list:

new_list = [[obj_1, obj_2], [obj_3], ...]

where obj_1.some_attr == obj_2.some_attr.

I could throw some for loops and if checks together, but this is ugly. Is there a pythonic way for this? by the way, the attributes of the objects are all strings.

Alternatively a solution for a list containing tuples (of the same length) instead of objects is appreciated, too.

Answer

S.Lott picture S.Lott · Jul 6, 2011

defaultdict is how this is done.

While for loops are largely essential, if statements aren't.

from collections import defaultdict


groups = defaultdict(list)

for obj in old_list:
    groups[obj.some_attr].append(obj)

new_list = groups.values()

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