What causes the release of GIL in Python when working with threads?

In Python, the GIL (Global Interpreter Lock) is a mechanism that allows only one thread to execute Python bytecode at any given time.

The GIL is released in the following cases:

• During input/output operations (e.g., reading/writing files, network operations), where the thread may wait, and the GIL is released for other threads.
• When calling C extensions that explicitly release the GIL during long computations (e.g., NumPy, some libraries).
• When calling functions like time.sleep() or other blocking operations.

Thus, the GIL is released when a thread performs operations that do not require active execution of Python code, allowing other threads to run in parallel.

Example of releasing the GIL in a C extension:

Py_BEGIN_ALLOW_THREADS
// Long operation without Python API calls
Py_END_ALLOW_THREADS

This helps improve the performance of multi-threaded programs despite the presence of the GIL.