In issue 21527 <http://bugs.python.org/issue21527> the concurrent.futures.ThreadPoolExecutor was changed to have a default value for max_workers. When asyncio.base_events.BaseEventLoop.run_in_executor creates a default ThreadPoolExecutor it specifies a value of 5 for max_workers, presumably because at the time it was written ThreadPoolExecutor didn't have a default for max_workers. This is confusing because on reading the documentation for ThreadPoolExecutor one might assume that the default specified there is what will be used if a default executor isn't supplied via BaseEventLoop.set_default_executor.

I propose that BaseEventLoop.run_in_executor be changed to not supply a default for max_workers. If this isn't acceptable, a note ought to be put in the run_in_executor documentation.