Test of Python threading fails with extra calls to mock'd thread target

----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/afletcher/thread/tests/test_tt.py", line 30, in test_timer_thread
    tt.timer_thread.assert_has_calls(master_calls)
  File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/unittest/mock.py", line 824, in assert_has_calls
    ) from cause
AssertionError: Calls not found.
Expected: [call(1), call(2), call(3), call(4), call(5), call(6), call(7), call(8), call(9)]
Actual: [call.__bool__(),
 call(1),
 call.__bool__(),
 call(2),
 call.__bool__(),
 call(3),
 call.__bool__(),
 call(4),
 call.__bool__(),
 call(5),
 call.__bool__(),
 call(6),
 call.__bool__(),
 call(7),
 call.__bool__(),
 call(8),
 call.__bool__(),
 call(9)]

test_tt.py

import tt

from importlib import reload
import unittest
from unittest.mock import patch, MagicMock, call

class TestTT(unittest.TestCase):
    def setUp(self):
        reload(tt)
        
    def test_main(self):
        tt.master_thread = MagicMock()

        master_calls = [
            call(),
        ]
        tt.main()
        tt.master_thread.assert_has_calls(master_calls)
    def test_timer_thread(self):
        tt.timer_thread = MagicMock()

        master_calls = [
            call(1),
            call(2),
            call(3),
            call(4),
            call(5),
            call(6),
            call(7),
            call(8),
            call(9),
        ]
        tt.main()
        tt.timer_thread.assert_has_calls(master_calls)

tt.py

from threading import Thread
from time import sleep
import sys

def timer_thread(t):
    for i in range(1, 10):
        sleep(1)
        print(t, i)

def master_thread():
    ts = []
    for t in range(1,10):
        ts.append(Thread(target=timer_thread, args=(t,)))

    for t in ts:
        t.start()
    for t in ts:
        t.join()

def main():
    print("Hello")
    m = Thread(target=master_thread)
    m.start()
    m.join()
    print("world")

if __name__ == '__main__':
    sys.exit(main())

python3 -m unittest discover test_tt.py

Why does the test of timer_thread fail? I asked this question to some friends and vishnubob and jwm both pointed out that it's probably because Thread is testing if the target callable is valid, which results in seeing extra calls to the mock'd object's __bool__() - and this is indeed true, according to the python source. Mystery solved!