What is the role of GIL in Python and why can it limit multithreading?

GIL (Global Interpreter Lock) is a global lock for the Python interpreter that ensures only one thread executes Python bytecode at any given time.

Role of GIL:

• Ensures the safety of the interpreter's internal state, simplifying implementation.
• Prevents multiple threads from accessing Python objects simultaneously, reducing the likelihood of errors.

Why GIL limits multithreading:

• Even if a program has multiple threads, they cannot execute Python code concurrently, which reduces efficiency on multi-core processors.
• Multithreading in Python with GIL is useful for I/O operations but not for CPU-intensive tasks.

To bypass GIL limitations, use:

• Multiprocessing (the multiprocessing module).
• C extensions that release the GIL during heavy computations.

Example:

import threading

def cpu_bound():
    count = 0
    for i in range(10**7):
        count += i

threads = [threading.Thread(target=cpu_bound) for _ in range(4)]
for t in threads:
    t.start()
for t in threads:
    t.join()

Despite having 4 threads, due to GIL, this code will not run faster than in a single thread.