In what case does it make sense to use multithreading in Python despite GIL?

Despite the presence of GIL (Global Interpreter Lock) in CPython, multithreading in Python makes sense to use in tasks where the main time is spent on input/output (I/O) operations, rather than on computations.

Examples of such tasks:

• Network requests (HTTP, databases)
• Reading and writing files
• Interacting with external devices

In these scenarios, threads can work in parallel, waiting for I/O operations to complete, which increases the overall performance and responsiveness of the program.

If the task is CPU-bound (requires intensive computations), then due to GIL, threads will not be able to effectively utilize multiple processor cores. In such cases, it is better to use multiprocessing (the multiprocessing module) or alternative Python implementations without GIL.

Example of using threads for I/O:

import threading
import requests

def fetch_url(url):
    response = requests.get(url)
    print(f"{url}: {len(response.content)} bytes")

urls = ["https://example.com", "https://python.org", "https://github.com"]
threads = []

for url in urls:
    thread = threading.Thread(target=fetch_url, args=(url,))
    thread.start()
    threads.append(thread)

for thread in threads:
    thread.join()

Here, threads allow parallel execution of network requests despite the GIL.