a.py

"""background module, first imported

Defines a `schedule` function for calling in a background thread.
"""
from __future__ import print_function
import os
import struct
import select
import sys
from threading import Thread

# pipe for signaling events
r, w = os.pipe()
r = os.fdopen(r, 'rb')
w = os.fdopen(w, 'wb')

evts = {}
n = 0

def schedule(f):
    """Schedule f to be called in the background thread"""
    global n
    evt_id = n
    n += 1
    evts[evt_id] = f
    w.write(struct.pack('H', evt_id))
    w.flush()
    print('sent', evt_id)


def background():
    """Eventloop for the background thread"""
    while True:
        rlist, _, _ = select.select([r], [], [], timeout=0.1)
        if not rlist:
            print('.', end='')
            sys.stdout.flush()
            continue
        evt_b = r.read(2)
        evt_id = struct.unpack('H', evt_b)[0]
        print('handling %i' % evt_id)
        f = evts.pop(evt_id)
        f()


t = Thread(target=background)
t.daemon = True
t.start()

b.py

"""
Second module that tries to synchronize with a's background thread
by passing `evt.set` to be called in the background and waiting with `evt.wait`.
"""

from __future__ import print_function

from a import schedule
from threading import Event

print("scheduling one, two, three")
schedule(lambda: print('one'))
schedule(lambda: print('two'))
schedule(lambda: print('three'))

evt = Event()
schedule(evt.set)
print('waiting')
if evt.wait(timeout=2):
    print('evt.set called (import)')
else:
    print("evt.set never called (import)")

test.py

"""test script

1. `import a` to set up
2. verify that scheduling works
3. `import b` and verify that scheduling *doesn't* work during import
4. verify that scheduling still works now that we are back in the main thread
"""
from __future__ import print_function
from threading import Event
import time

print("importing a")
import a

time.sleep(1)

print("Thread ready?")
a.schedule(lambda: print("ready"))

time.sleep(1)

print("importing b")
import b
print("imported b")
time.sleep(1)

# wait again, not during import
evt = Event()
a.schedule(evt.set)
print('waiting again')
if evt.wait(timeout=2):
    print('evt.set called (main)')
else:
    print("evt.set never called (main)")