How to use Python multiprocessing Pool.map to fill numpy array in a for loop
The following works. First it is a good idea to protect the main part of your code inside a main block in order to avoid weird side effects. The result of poo.map()
is a list containing the evaluations for each value in the iterator list_start_vals
, such that you don't have to create array_2D
before.
import numpy as npfrom multiprocessing import Pooldef fill_array(start_val): return list(range(start_val, start_val+10))if __name__=='__main__': pool = Pool(processes=4) list_start_vals = range(40, 60) array_2D = np.array(pool.map(fill_array, list_start_vals)) pool.close() # ATTENTION HERE print array_2D
perhaps you will have trouble using pool.close()
, from the comments of @hpaulj you can just remove this line in case you have problems...
If you still want to use the array fill, you can use pool.apply_async
instead of pool.map
. Working from Saullo's answer:
import numpy as npfrom multiprocessing import Pooldef fill_array(start_val): return range(start_val, start_val+10)if __name__=='__main__': pool = Pool(processes=4) list_start_vals = range(40, 60) array_2D = np.zeros((20,10)) for line, val in enumerate(list_start_vals): result = pool.apply_async(fill_array, [val]) array_2D[line,:] = result.get() pool.close() print array_2D
This runs a bit slower than the map
. But it does not produce a runtime error like my test of the map version: Exception RuntimeError: RuntimeError('cannot join current thread',) in <Finalize object, dead> ignored
The problem is due to running the pool.map
in for loop , The result of the map() method is functionally equivalent to the built-in map(), except that individual tasks are run parallel.so in your case the pool.map(fill_array,list_start_vals) will be called 20 times and start running parallel for each iteration of for loop , Below code should work
Code:
#!/usr/bin/pythonimport numpyfrom multiprocessing import Pooldef fill_array(start_val): return range(start_val,start_val+10)if __name__ == "__main__": array_2D = numpy.zeros((20,10)) pool = Pool(processes = 4) list_start_vals = range(40,60) # running the pool.map in a for loop is wrong #for line in xrange(20): # array_2D[line,:] = pool.map(fill_array,list_start_vals) # get the result of pool.map (list of values returned by fill_array) # in a pool_result list pool_result = pool.map(fill_array,list_start_vals) # the pool is processing its inputs in parallel, close() and join() #can be used to synchronize the main process #with the task processes to ensure proper cleanup. pool.close() pool.join() # Now assign the pool_result to your numpy for line,result in enumerate(pool_result): array_2D[line,:] = result print array_2D