before being passed to a child process. This argument is only
        used by ProcessPoolExecutor; it is ignored by
        ThreadPoolExecutor.

Returns:
    An iterator equivalent to: map(func, *iterables) but the calls may
    be evaluated out-of-order.

Raises:
    TimeoutError: If the entire result iterator could not be generated
        before the given timeout.
    Exception: If fn(*args) raises for any values.
c