What is the GIL (Global Interpreter Lock)?

Question

Accepted Answer

**GIL** is a mutex that allows only **one thread to execute Python bytecode** at a time.

**What it means:**
- In CPython, only one thread executes Python code at a time.
- Multithreading does NOT speed up CPU-bound tasks.
- I/O-bound tasks (network, disk) are not affected.

**Why does it exist?**
- Memory management (reference counting) is not thread-safe.
- Simplicity of implementation.

**How to bypass:**
- **Multiprocessing** — separate processes (each has its own GIL).
- **asyncio** — single-threaded concurrency.
- **C extensions** — release GIL (NumPy).

```python
# Multiprocessing
from multiprocessing import Pool

def cpu_task(x):
    return x ** 2

with Pool(4) as p:
    results = p.map(cpu_task, range(100))
```

**Note:** GIL is only in CPython. Other implementations (Jython, IronPython) don't have it.

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