multiprocessing returns “too many open files” but using `with…as` fixes it. Why?

You’re creating new processes inside a loop, and then forgetting to close them once you’re done with them. As a result, there comes a point where you have too many open processes. This is a bad idea.

You could fix this by using a context manager which automatically calls pool.terminate, or manually call pool.terminate yourself. Alternatively, why don’t you create a pool outside the loop just once, and then send tasks to the processes inside?

pool = multiprocessing.Pool(nprocess) # initialise your pool
for nprocess in process_per_cycle:
    ...       
    pool.map(cycle, offsets) # delegate work inside your loop

pool.close() # shut down the pool

For more information, you could peruse the multiprocessing.Pool documentation.