Attached file runs fine. Uncommenting line 19 makes python crash. Also, no crash happens if order of execution of map() and executor.map() is switched.

This problem seems to be related to numpy in some way, the code does not crash if the commented return of function user_square is substituted for numpy functions.

Using a mac with Yosemite 10.10.3. When program crashed I get window that says:

"
Python quit unexpectedly.

Click Reopen to open the application again. Click Report to see more detailed information and send a report to Apple.
"

This is the traceback:

Traceback (most recent call last):
  File "processExample.py", line 27, in <module>
    main()
  File "processExample.py", line 22, in main
    result3 = list(executor.map(user_square, nums))
  File "/usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/concurrent/futures/_base.py", line 549, in result_iterator
    yield future.result()
  File "/usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/concurrent/futures/_base.py", line 402, in result
    return self.__get_result()
  File "/usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/concurrent/futures/_base.py", line 354, in __get_result
    raise self._exception
concurrent.futures.process.BrokenProcessPool: A process in the process pool was terminated abruptly while the future was running or pending.