What limits the possibility of full multithreading in Python and is it related to GIL?

The ability for full multithreading in Python is limited due to the GIL (Global Interpreter Lock) — a global lock for the interpreter. The GIL ensures that only one thread executes Python bytecode at a time, preventing issues with concurrent access to Python objects.

This means that even if a program has multiple threads, they cannot execute Python code simultaneously on multiple CPU cores, which reduces the efficiency of multithreading for CPU-bound tasks.

However, the GIL does not prevent threads from performing input-output operations in parallel, so multithreading is effective for I/O-bound tasks.

To bypass GIL limitations, the following are used:

• Multiprocessing (the multiprocessing module), where each process has its own interpreter and GIL.
• Using C extensions that release the GIL during heavy computations.

Thus, the GIL is a key limitation for multithreading in CPython.