python - What is itertools.groupby() used for?

Question

While reading the python documentation I came across the itertools.groupby() function. It was not very straightforward so I decided to look up some info here on stackoverflow. I found something from How do I use Python's itertools.groupby()?.

There seems to be little info about it here and in the documentation so I decided I to post my observations for comments.

Thanks

Answer 1

To start with, you may read the documentation here.

I will place what I consider to be the most important point first. I hope the reason will become clear after the examples.

ALWAYS SORT ITEMS WITH THE SAME KEY TO BE USED FOR GROUPING SO AS TO AVOID UNEXPECTED RESULTS

itertools.groupby(iterable, key=None or some func) takes a list of iterables and groups them based on a specified key. The key specifies what action to apply to each individual iterable, the result of which is then used as the heading for each grouping the items; items which end up having same 'key' value will end up in the same group.

The return value is an iterable similar to a dictionary in that it is of the form {key : value}.

Example 1

# note here that the tuple counts as one item in this list. I did not
# specify any key, so each item in the list is a key on its own.
c = groupby(['goat', 'dog', 'cow', 1, 1, 2, 3, 11, 10, ('persons', 'man', 'woman')])
dic = {}
for k, v in c:
    dic[k] = list(v)
dic

results in

{1: [1, 1],
 'goat': ['goat'],
 3: [3],
 'cow': ['cow'],
 ('persons', 'man', 'woman'): [('persons', 'man', 'woman')],
 10: [10],
 11: [11],
 2: [2],
 'dog': ['dog']}

Example 2

# notice here that mulato and camel don't show up. only the last element with a certain key shows up, like replacing earlier result
# the last result for c actually wipes out two previous results.

list_things = ['goat', 'dog', 'donkey', 'mulato', 'cow', 'cat', ('persons', 'man', 'woman'), 
               'wombat', 'mongoose', 'malloo', 'camel']
c = groupby(list_things, key=lambda x: x[0])
dic = {}
for k, v in c:
    dic[k] = list(v)
dic

results in

{'c': ['camel'],
 'd': ['dog', 'donkey'],
 'g': ['goat'],
 'm': ['mongoose', 'malloo'],
 'persons': [('persons', 'man', 'woman')],
 'w': ['wombat']}

Now for the sorted version

 # but observe the sorted version where I have the data sorted first on same key I used for grouping
list_things = ['goat', 'dog', 'donkey', 'mulato', 'cow', 'cat', ('persons', 'man', 'woman'), 
               'wombat', 'mongoose', 'malloo', 'camel']
sorted_list = sorted(list_things, key = lambda x: x[0])
print(sorted_list)
print()
c = groupby(sorted_list, key=lambda x: x[0])
dic = {}
for k, v in c:
    dic[k] = list(v)
dic

results in

['cow', 'cat', 'camel', 'dog', 'donkey', 'goat', 'mulato', 'mongoose', 'malloo', ('persons', 'man', 'woman'), 'wombat']
{'c': ['cow', 'cat', 'camel'],
 'd': ['dog', 'donkey'],
 'g': ['goat'],
 'm': ['mulato', 'mongoose', 'malloo'],
 'persons': [('persons', 'man', 'woman')],
 'w': ['wombat']}

Example 3

things = [("animal", "bear"), ("animal", "duck"), ("plant", "cactus"), ("vehicle", "harley"), 
          ("vehicle", "speed boat"), ("vehicle", "school bus")]
dic = {}
f = lambda x: x[0]
for key, group in groupby(sorted(things, key=f), f):
    dic[key] = list(group)
dic

results in

{'animal': [('animal', 'bear'), ('animal', 'duck')],
 'plant': [('plant', 'cactus')],
 'vehicle': [('vehicle', 'harley'),
  ('vehicle', 'speed boat'),
  ('vehicle', 'school bus')]}

Now for the sorted version. I changed the tuples to lists here. Same results either way.

things = [["animal", "bear"], ["animal", "duck"], ["vehicle", "harley"], ["plant", "cactus"], 
          ["vehicle", "speed boat"], ["vehicle", "school bus"]]
dic = {}
f = lambda x: x[0]
for key, group in groupby(sorted(things, key=f), f):
    dic[key] = list(group)
dic

results in

{'animal': [['animal', 'bear'], ['animal', 'duck']],
 'plant': [['plant', 'cactus']],
 'vehicle': [['vehicle', 'harley'],
  ['vehicle', 'speed boat'],
  ['vehicle', 'school bus']]}