What's the idiomatic Python equivalent to Django's 'regroup' template tag?

Question

http://docs.djangoproject.com/en/dev/ref/templates/builtins/#regroup

I can think of a few ways of doing it with loops but I'd particularly like to know if there is a neat one-liner.

Answer 1

Combine itertools.groupby with operator.itemgetter to get a pretty nice solution:

from operator import itemgetter
from itertools import groupby

key = itemgetter('gender')
iter = groupby(sorted(people, key=key), key=key)

for gender, people in iter:
    print '===', gender, '==='
    for person in people:
        print person

Answer 2

If the source of data (people in this case) is already sorted by the key, you can bypass the sorted call:

iter = groupby(people, key=lambda x:x['gender'])
for gender, people in iter:
    print '===', gender, '==='
    for person in people:
        print person

Note: If sorted is a common dictionary, there are no guarantees of order; therefore you must call sorted. Here I'm supposing that sorted is a collections.OrderedDict or some other kind of ordered data structure.