Counting Python threads vs C/C++ threads

[Thomas Jollans]

On 17/07/2019 09.58, Barry Scott wrote:
>
>> On 16 Jul 2019, at 20:48, Dan Stromberg <drsalists at gmail.com> wrote:
>>
>>
>>  
>> A question arises though: Does threading.active_count() only show Python threads created with the threading module?  What about threads created with the thread module?
> Only pythons threads, if you think about it why would python care about threads it does not control?


As the docs say, this counts threading.Thread objects. It does not count
all threads started from Python: threads started with the _thread
module, for instance, are not included.

What is more, threads started in external libraries can acquire the GIL
and run Python code. A great example of this are QThreads in a PyQt5
application: QThreads are started by the Qt runtime, which calls a Qt
slot. This Qt slot then might happen to be implemented in Python. I'm
sure other libraries do similar things.

Example with _thread just to check active_count behaviour:

#!/usr/bin/env python3

import threading
import _thread
import time

def thread_func(i):
    print('Starting thread', i)
    time.sleep(0.5)
    print('Thread done', i)

print('Using threading.Thread')
t1 = threading.Thread(target=thread_func, args=(1,))
t1.start()
time.sleep(0.1)
print('active threads:', threading.active_count())
t1.join()


print('Using threading & _thread')
t1 = threading.Thread(target=thread_func, args=(1,))
t1.start()
t2_id = _thread.start_new_thread(thread_func, (2,))
time.sleep(0.1)
print('active threads:', threading.active_count())
time.sleep(0.6)
print('Done, hopefully')