Python collections: Counter, defaultdict, deque

Counter — Fast Counting

from collections import Counter
words = "the quick brown fox jumps over the lazy dog the the".split()
print(Counter(words).most_common(3))
# [('the', 4), ('quick', 1), ('brown', 1)]

defaultdict — No KeyError

from collections import defaultdict
groups = defaultdict(list)
for word in ["apple", "ant", "bird", "bee"]:
    groups[word[0]].append(word)
# {'a': ['apple','ant'], 'b': ['bird','bee']}

deque — Double-Ended Queue

from collections import deque
dq = deque([1, 2, 3])
dq.appendleft(0)   # [0,1,2,3]
dq.append(4)       # [0,1,2,3,4]
dq.popleft()       # O(1) — list.pop(0) is O(n)

When to Reach For These

• Counting anything → Counter.
• Grouping by key → defaultdict(list).
• Queue/BFS → deque.