collections Module

Introduction

collections ships production-grade containers: Counter, defaultdict, deque, OrderedDict, namedtuple. Use them instead of reinventing.

Understanding the topic

Core concepts to understand:

• Counter — frequency map.
• defaultdict — auto-init missing keys.
• deque — O(1) prepend/append.
• namedtuple — tuple with names.

Syntax reference

Visual flow / code:

from collections import Counter, defaultdict, deque

# Counter
Counter("mississippi").most_common(2)
# [('i', 4), ('s', 4)]

# defaultdict
groups = defaultdict(list)
for word in ["apple", "ant", "bee"]:
    groups[word[0]].append(word)

# deque (efficient queue)
q = deque(maxlen=3)
q.append(1); q.append(2); q.appendleft(0)

Execution workflow

Real-world use

Counter is the one-liner for word/event histograms. deque is the right structure for rolling windows and BFS queues.

Best practices

• Counter for histograms.
• defaultdict for grouped data.
• deque for FIFO/sliding windows.

Hands-on exercise

Interview preparation — practice these questions:

• When use deque vs list?
• What does Counter.most_common return?
• defaultdict use case?

Summary

In summary: collections = batteries. Use them — don't reinvent.