ASYNCIO.LOCK

Why Does Python Need asyncio.Lock? INTRODUCTION After understanding asyncio.Semaphore , I thought I had learned everything required to control multiple coroutines. A semaphore limits how many coroutines can execute simultaneously. Then another question came to my mind. If Python's event loop executes only one coroutine at a time, why do we even need a Lock? Initially, I assumed a lock was unnecessary because there was only one thread. But after experimenting with shared variables, I realized that even though only one coroutine executes at a particular instant, multiple coroutines can still interfere with each other. In this article, I'll explain the problem that led to asyncio.Lock , how it works, and why almost every backend application uses it. What You Will Learn Why asyncio.Lock exists What is a race condition What is a critical section How Lock works internally Practical examples Real-world backend use cases Prerequisites Before learning asyncio.Lock , you should understand: Coroutines Event Loop await asyncio.Semaphore The Problem Suppose we have a shared variable. counter = 0 Now imagine two coroutines trying to increment it. async def increment (): global counter temp = counter await asyncio . sleep ( 1 ) counter = temp + 1 Initially I expected the final value to become 2 because two coroutines are incrementing the counter. But that wasn't what happened. Let's See What Actually Happens Initially counter = 0 Now Coroutine A starts executing. Read counter ↓ temp = 0 ↓ await The coroutine reaches await . The event loop suspends it and starts another coroutine. Now Coroutine B executes. Read counter ↓ temp = 0 ↓ await Notice something interesting. Both coroutines have already read counter = 0 Now Coroutine A resumes. counter = 1 Then Coroutine B resumes. counter = 1 The final value becomes 1 instead of 2 This is called a Race Condition . Why Did This Happen? Initially I blamed the Event Loop. Later I realized, the Event Loop didn't do anything wrong. Its job is