Using Pool.map with a list : TypeError: '<=' not supported between instances of 'list' and 'int'

Question

I m trying to accelerate my code so I use :

pool = Pool(cpu_count()-1)
print('Start multi process')
res = pool.map(max_def_in_circle, range(len(dataT)), All_index_in_c)

with All_index_in_c is a list of list : here a preview

[[2, 12], [11, 25, 26, 27, 28, 29], [0, 3, 12], [], [21, 22, 44, 45, 46], .... ]

and max_def_in_circle a very basic function which works fine

But when I run this I got this error message :

<ipython-input-18-3bd316855b1c> in <module>
      6 # pool.join()
      7 print('Start multi process')
----> 8 res = pool.map(max_def_in_circle, range(len(dataT)), All_index_in_c)

/usr/lib/python3.8/multiprocessing/pool.py in map(self, func, iterable, chunksize)
    362         in a list that is returned.
    363         '''
--> 364         return self._map_async(func, iterable, mapstar, chunksize).get()
    365 
    366     def starmap(self, func, iterable, chunksize=None):

/usr/lib/python3.8/multiprocessing/pool.py in _map_async(self, func, iterable, mapper, chunksize, callback, error_callback)
    483 
    484         task_batches = Pool._get_tasks(func, iterable, chunksize)
--> 485         result = MapResult(self, chunksize, len(iterable), callback,
    486                            error_callback=error_callback)
    487         self._taskqueue.put(

/usr/lib/python3.8/multiprocessing/pool.py in __init__(self, pool, chunksize, length, callback, error_callback)
    795         self._value = [None] * length
    796         self._chunksize = chunksize
--> 797         if chunksize <= 0:
    798             self._number_left = 0
    799             self._event.set()

TypeError: '<=' not supported between instances of 'list' and 'int'

But When I did my search on this problem, several people seem able to use list in pool.map I don't understand why, maybe it is a version problem ? If someone can help me, Thanks

Answer 1

As @DarrylG mentioned, pool.map supports functions of one argument only. Use pool.starmap to pass multiple arguments to max_def_in_circle, e.g.

res = pool.starmap(max_def_in_circle, zip(range(len(dataT)), All_index_in_c))

The zip function will output the entries in each list as pairs. See here for details.