I understand your hesitation to add a fake. Would it be better to make it possible to subclass Executor so that a third party implementation of this can be developed?

As for an example, here is an example of nondeterminism when using a ThreadPoolExecutor with a single worker. It sometimes prints "False" and sometimes "True" on my machine.

from concurrent import futures
import time

complete = False

def complete_eventually():
  global complete
  for _ in range(150000):
    pass
  complete = True

with futures.ThreadPoolExecutor(max_workers=1) as pool:
  pool.submit(complete_eventually)
  print(complete)