Locks (Lock vs RLock)

Lock

A LockLock allows only one thread to enter a critical section at a time.

lock_example.py

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

lock_example.py

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

What is a “critical section”?

A piece of code that must not be interrupted by another thread while it updates shared data.

RLock (re-entrant lock)

An RLockRLock can be acquired multiple times by the same thread.

Use it when:

a function that holds a lock calls another function that tries to acquire the same lock

rlock_example.py

import threading
 
lock = threading.RLock()
 
 
def outer():
    with lock:
        inner()
 
 
def inner():
    with lock:
        print("safe")
 
 
t = threading.Thread(target=outer)
t.start(); t.join()

rlock_example.py

import threading
 
lock = threading.RLock()
 
 
def outer():
    with lock:
        inner()
 
 
def inner():
    with lock:
        print("safe")
 
 
t = threading.Thread(target=outer)
t.start(); t.join()

Best practices

Keep with lock:with lock: blocks short.
Always acquire locks in a consistent order if using multiple locks.

🧪 Try It Yourself

Exercise 1 – Basic Lock Usage

Exercise 2 – Lock as Context Manager

Exercise 3 – Detect Locked State

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