Thread Synchronization with Lock

The problem: race conditions

A race condition happens when two threads update shared state at the same time.

Example: incrementing a shared counter.

race_condition.py

import threading
 
counter = 0
 
 
def inc():
    global counter
    for _ in range(100_000):
        counter += 1
 
 
t1 = threading.Thread(target=inc)
t2 = threading.Thread(target=inc)
 
t1.start(); t2.start()
t1.join(); t2.join()
 
print("counter:", counter)

race_condition.py

import threading
 
counter = 0
 
 
def inc():
    global counter
    for _ in range(100_000):
        counter += 1
 
 
t1 = threading.Thread(target=inc)
t2 = threading.Thread(target=inc)
 
t1.start(); t2.start()
t1.join(); t2.join()
 
print("counter:", counter)

You might expect 200000, but results can vary.

Fix with Lock

lock_fix.py

import threading
 
counter = 0
lock = threading.Lock()
 
 
def inc():
    global counter
    for _ in range(100_000):
        with lock:
            counter += 1
 
 
t1 = threading.Thread(target=inc)
t2 = threading.Thread(target=inc)
 
t1.start(); t2.start()
t1.join(); t2.join()
 
print("counter:", counter)

lock_fix.py

import threading
 
counter = 0
lock = threading.Lock()
 
 
def inc():
    global counter
    for _ in range(100_000):
        with lock:
            counter += 1
 
 
t1 = threading.Thread(target=inc)
t2 = threading.Thread(target=inc)
 
t1.start(); t2.start()
t1.join(); t2.join()
 
print("counter:", counter)

Best practices

Keep locked sections small.
Prefer with lock:with lock: so you don’t forget to release.
Avoid deadlocks (don’t acquire multiple locks in random order).

🧪 Try It Yourself

Exercise 1 – Basic Lock Usage

Exercise 2 – Lock as Context Manager

Exercise 3 – Detect Locked State

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