Threads vs. Processes

A thread is a separate line of execution inside the same program, sharing memory with other threads. A process is a fully separate program instance, with its own memory.

Python has a detail worth knowing here: the Global Interpreter Lock (GIL) means only one thread runs Python bytecode at a time, even on a multi-core machine. Threads still help for I/O-bound work (waiting on files/network), but don't give real parallelism for CPU-heavy work — use multiprocessing (separate processes, each with its own GIL) for that instead.

from multiprocessing import Pool

def square(n):
    return n * n

with Pool(4) as pool:
    print(pool.map(square, [1, 2, 3, 4]))

Expected output

[1, 4, 9, 16]