Behavior of @register in Python

main.py

import os
import threading
from multiprocessing import Pool
from atexit import register
from sys import exit
from typing import Union

@register
def term():
    print("TERM!")

@register
def term2():
    print("TERM2!")

@register
def term3():
    print("TERM3!")

def f(x: int, source: str):
    @register
    def _():
        print(f"TERM! f(x={x}, source='{source}')")

    print(f"PID: {os.getpid()}")

    return x * x

class FooThread(threading.Thread):
    _return: int

    def __init__(self, x):
        threading.Thread.__init__(self)
        self.x = x

    def run(self):
        self._return = f(self.x, "thread")

    def join(self, timeout: Union[float, None] = None):
        threading.Thread.join(self, timeout)
        return self._return


if __name__ != "__main__":
    print("-not __main__-")
    exit(0)

print("-using Pool-")
with Pool(5) as p:
    print(p.starmap(f, [(i, "pool") for i in range(1, 4)]))

print("-using FooThread-")
threads = [FooThread(i) for i in range(1, 4)]
[thread.start() for thread in threads]
print([thread.join() for thread in threads])

print("-done-")