Hum, I should elaborate my explanation :-) os.fork() removes all threads except of the current thread. It's clearly stated in the fork manual page, but the Python os.fork() doesn't say anything about threads.
https://docs.python.org/dev/library/os.html#os.fork

Short example:
---
import os, threading, time

t = threading.Thread(target=time.sleep, args=(3.0,))
t.start()

print("First process", threading.enumerate())

pid = os.fork()
if pid != 0:
    os.waitpid(pid, 0)
else:
    print("Child process", threading.enumerate())
---

Output:
---
First process [<_MainThread(MainThread, started 140737353955072)>, <Thread(Thread-1, started 140737216849664)>]
Child process [<_MainThread(MainThread, started 140737353955072)>]
---

The thread is removed in the child process.

Again, *don't create threads before fork*. More generally, don't create any resource before fork: don't open files, don't create locks, don't open a connection to a database, don't start an event loop, etc.