What is the Global Interpreter Lock (GIL) in Python?

The Global Interpreter Lock (GIL) in Python is a mutex that allows only one thread to execute Python bytecode at a time, ensuring thread safety with reference counting. It simplifies memory management but prevents true parallelism in CPU-bound multi-threaded programs.

The GIL is a global lock that serializes execution of Python bytecode across threads, which prevents concurrent execution in multi-threaded CPU-heavy programs but does not affect I/O-bound concurrency much. It was introduced to simplify internal memory management and avoid race conditions.

Code

import threading

x = 0

def increment():
    global x
    for _ in range(1000000):
        x += 1

t1 = threading.Thread(target=increment)
t2 = threading.Thread(target=increment)
t1.start()
t2.start()
t1.join()
t2.join()
print(x)