test_python_multiprocessing.py

import multiprocessing

def functionality(arg1):
	import sys
	print("arg1: " + str(arg1))
	print("sys.executable: " + str(sys.executable))
	print("-----\n")
	return sys.executable

def worker(queue, arg1):
	import traceback
	try:
		retval = functionality(arg1)
		queue.put({"retval": retval})
	except Exception as e:
		queue.put({"exception": e, "traceback_str": traceback.format_exc()})
		raise
	finally:
		pass

def spawn_worker(arg1):
	queue = multiprocessing.Queue()
	p = multiprocessing.Process(target=worker, args=(queue, arg1,))
	p.start()
	p.join()
	
	err_or_ret = queue.get()

	handle_worker_err(err_or_ret)
	if p.exitcode != 0:
		raise RuntimeError("Subprocess failed with code " + str(p.exitcode) + ", but no exception was thrown.")
	return err_or_ret["retval"]

def handle_worker_err(err_or_ret):
	if "retval" in err_or_ret:
		return None
	err = err_or_ret
	#import traceback
	if (err is not None):
		#traceback.print_tb(err["traceback"]) # TODO use e.g. tblib to get traceback
		print("The exception was thrown in the child process, reraised in parent process:")
		print(err["traceback_str"])
		raise err["exception"]

def test_exes():
	exe1 = functionality("called directly")
	exe2 = spawn_worker("called via multiprocessing")

	print("The two exes:")
	print(exe1)
	print(exe2)

	assert exe1 == exe2

if __name__ == "__main__":
	test_exes()

The output of running the script in a version of python that comes with a scriptable application:

arg1: called directly
sys.executable: C:\[virtual_environment_path]\Scripts\python.exe
-----

arg1: called via multiprocessing
sys.executable: C:\[application_with_python]\contrib\Python37\python.exe
-----

The two exes:
C:\[virtual_environment_path]\Scripts\python.exe
C:\[application_with_python]\contrib\Python37\python.exe
Traceback (most recent call last):
  File ".\test_python_multiprocessing.py", line 67, in <module>
    test_exes()
  File ".\test_python_multiprocessing.py", line 64, in test_exes
    assert exe1 == exe2
AssertionError