Lesson 2 — Queue

Queue: first in, first out

A queue is like a line at a shop: people join at the back and are served from the front — FIFO (first in, first out). Removing from the front of a normal list is slow on big lists, so Python offers deque ("deck", a double-ended queue) in the collections module. append() adds at the back, popleft() removes from the front — both instantly.

from collections import deque
q = deque(["Ann", "Bob"])
q.popleft()   # "Ann"

Why it matters: printers, web servers and message systems all process jobs in arrival order with queues.