Python: How to group a list of objects by their characteristics or attributes? [duplicate]

Question

I want to separate a list of objects into sublists, where objects with same attribute/characteristic stay in the same sublist.

Suppose we have a list of strings:

["This", "is", "a", "sentence", "of", "seven", "words"]

We want to separate the strings based on their length as follows:

[['sentence'], ['a'], ['is', 'of'], ['This'], ['seven', 'words']]

The program I currently come up with is this

sentence = ["This", "is", "a", "sentence", "of", "seven", "words"]
word_len_dict = {}
for word in sentence:
    if len(word) not in word_len_dict.keys():
        word_len_dict[len(word)] = [word]
    else:
        word_len_dict[len(word)].append(word)


print word_len_dict.values()

I want to know if there is a better way to achieve this?

Answer 1

With defaultdict(list), you can omit the key-existence check:

from collections import defaultdict

word_len_dict = defaultdict(list)

for word in sentence:
    word_len_dict[len(word)].append(word)

Answer 2

Take a look at itertools.groupby(). Note your list must be sorted first (more expensive than your method OP).

>>> from itertools import groupby
>>> l = ["This", "is", "a", "sentence", "of", "seven", "words"]
>>> print [list(g[1]) for g in groupby(sorted(l, key=len), len)]
[['a'], ['is', 'of'], ['This'], ['seven', 'words'], ['sentence']]

or if you want a dictionary ->

>>> {k:list(g) for k, g in groupby(sorted(l, key=len), len)}
{8: ['sentence'], 1: ['a'], 2: ['is', 'of'], 4: ['This'], 5: ['seven', 'words']}