python - How to use a generator as an iterable with Multiprocessing map function

Question

When I use a generator as an iterable argument with multiprocessing.Pool.map function:

pool.map(func, iterable=(x for x in range(10)))

It seems that the generator is fully exhausted before func is ever called.

I want to yield each item and pass it to each process, thanks

Answer 1

multiprocessing.map converts iterables without a __len__ method to a list before processing. This is done to aid the calculation of chunksize, which the pool uses to group worker arguments and reduce the round trip cost of scheduling jobs. This is not optimal, especially when chunksize is 1, but since map must exhaust the iterator one way or the other, its usually not a significant issue.

The relevant code is in pool.py. Notice its use of len:

def _map_async(self, func, iterable, mapper, chunksize=None, callback=None,
        error_callback=None):
    '''
    Helper function to implement map, starmap and their async counterparts.
    '''
    if self._state != RUN:
        raise ValueError("Pool not running")
    if not hasattr(iterable, '__len__'):
        iterable = list(iterable)

    if chunksize is None:
        chunksize, extra = divmod(len(iterable), len(self._pool) * 4)
        if extra:
            chunksize += 1
    if len(iterable) == 0:
        chunksize = 0