collections Module (deque, Counter, defaultdict)

Why collections

collections provides specialized data structures that are safer and often faster than rolling your own patterns.

deque for Queues

from collections import deque

q = deque(["job1", "job2"])
q.append("job3")
item = q.popleft()

Counter for Counting

from collections import Counter

c = Counter(["ok", "ok", "fail"])
print(c["ok"])

defaultdict for Grouping

from collections import defaultdict

groups = defaultdict(list)
groups["A"].append("alice")
groups["A"].append("bob")

Operational Checklist

• Use deque for queue-like workloads (popleft is efficient).
• Use Counter for counting outcomes and metrics snapshots.
• Use defaultdict for grouping to avoid key-existence boilerplate.

Failure Modes

• List as queue: pop(0) becomes slow under load.
• Manual counting bugs: missing initialization leads to KeyError or wrong totals.