The GIL, explained — what it is and when it actually matters

The Global Interpreter Lock (GIL) is a mutex in CPython that lets only one thread execute Python bytecode at a time. It simplifies memory management but means threads do not run Python code truly in parallel.

When the GIL hurts

It only bites CPU-bound work — heavy number crunching in pure Python across multiple threads will not speed up, because the threads take turns.

When it does not matter

For I/O-bound work (network calls, disk, databases), threads release the GIL while waiting, so threading and asyncio give real concurrency.

Working around it

• Use multiprocessing or concurrent.futures.ProcessPoolExecutor to get separate interpreters and real parallelism.
• Push hot loops into NumPy or C extensions, which release the GIL.
• Watch the free-threaded (no-GIL) builds landing in recent CPython releases.