collections Module

Python · Intermediate

Definition

collections offers Counter, defaultdict, deque, namedtuple and OrderedDict for common patterns.

Shortens DSA solutions dramatically.

Diagram

Flowchart

Example: Group anagrams

from collections import defaultdict
g = defaultdict(list)
for w in ['eat', 'tea', 'tan', 'nat']:
    g[''.join(sorted(w))].append(w)
print(list(g.values()))

Output: [['eat', 'tea'], ['tan', 'nat']]

Key points

Counter supports + and − of counts
heapq pairs well for top-k

Interview questions

dict vs defaultdict?

defaultdict creates a default value for missing keys instead of raising KeyError.