Atomic Operations

The Idea

An atomic operation cannot be split. Other threads see the world before it or after it, never in between, so no lock is needed. The catch is scope: atomicity covers one step, and most real invariants span several.

Real-World Example

The ticket dispenser at a deli counter. Pulling the tab tears your number and advances the roll in a single motion, so two customers reaching at once still leave with different numbers. Nobody can grab a half-torn 47.

The Code

import itertools, threading

dispenser = itertools.count(1)          # next() is one indivisible step
tickets = []

def take(n):
    for _ in range(n):
        tickets.append(next(dispenser))  # append is atomic too

crowd = [threading.Thread(target=take, args=(50,)) for _ in range(4)]
for person in crowd: person.start()
for person in crowd: person.join()
print(len(tickets), len(set(tickets)))   # 200 200 — no number issued twice

import itertools

dispenser = itertools.count(1)
served = [next(dispenser) for _ in range(3)]
served.append(next(dispenser))

print(served, len(set(served)))