list-dictionaries-group-by-key.py

import itertools
from operator import itemgetter
sorted_animals = sorted(animals, key=itemgetter('size'))

animals = [{'name':'cow', 'size':'large'},{'name':'bird', 'size':'small'},{'name':'fish', 'size':'small'},{'name':'rabbit', 'size':'medium'},{'name':'pony', 'size':'large'},{'name':'squirrel', 'size':'medium'},{'name':'fox', 'size':'medium'}]

for key, group in itertools.groupby(sorted_animals, key=lambda x:x['size']):
        print key,
        print list(group)
        
# large [{'name': 'cow', 'size': 'large'}, {'name': 'pony', 'size':'large'}]
# medium [{'name': 'rabbit', 'size': 'medium'}, {'name': 'squirrel', 'size':'medium'}, {'name': 'fox', 'size': 'medium'}]
# small [{'name': 'bird', 'size': 'small'}, {'name': 'fish', 'size': 'small'}]