TIL - collections.deque with maxlen is a Zero-Effort Rolling Window

Today I Learned May 9, 2026

A deque with maxlen automatically discards the oldest element when the capacity is full — no manual trimming, no index math.

from collections import deque

# Keep the last 5 sensor readings
window = deque(maxlen=5)

for reading in [1, 2, 3, 4, 5, 6, 7]:
    window.append(reading)

print(window)           # deque([3, 4, 5, 6, 7], maxlen=5)
print(sum(window) / 5)  # 5.0  — rolling average

# Also O(1) on both ends — faster than list.insert(0, x)
window.appendleft(99)
print(window)           # deque([99, 3, 4, 5, 6], maxlen=5)

deque appends and pops from both ends in O(1), making it the right choice for queues, sliding windows, and history buffers.