Python multiprocessing works in script but hangs in Windows Server

This can happen on Windows Server when the child Python processes are not starting cleanly or are waiting on something that exists only in the parent process. The if name == "main": guard is required, but in server environments you should also make sure the script is run as a real .py fil.

from multiprocessing import Pool, freezesupport
def worker(x):
    return x * x
if name == "main":
    freezesupport()
    with Pool(processes=4) as p:
        result = p.map(worker, range(10))
    print(result)

If this still hangs, test with Pool(processes=1), check Task Manager to see whether child python.exe processes are created, run from an elevated command prompt, temporarily exclude the script folder from antivirus.