Sharing mutable global variable in Python multiprocessing.Pool

Question

I'm trying to update a shared object (a dict) using the following code. But it does not work. It gives me the input dict as an output.

Edit: Exxentially, What I'm trying to achieve here is to append items in the data (a list) to the dict's list. Data items give indices in the dict.

Expected output: {'2': [2], '1': [1, 4, 6], '3': [3, 5]}
Note: Approach 2 raise error TypeError: 'int' object is not iterable

1. Approach 1

   from multiprocessing import *
   def mapTo(d,tree):
           for idx, item in enumerate(list(d), start=1):
               tree[str(item)].append(idx)
   
   data=[1,2,3,1,3,1]
   manager = Manager()
   sharedtree= manager.dict({"1":[],"2":[],"3":[]})
   with Pool(processes=3) as pool:
       pool.starmap(mapTo, [(data,sharedtree ) for _ in range(3)])
   

2. Approach 2

 from multiprocessing import *
 def mapTo(d):
         global tree
         for idx, item in enumerate(list(d), start=1):
             tree[str(item)].append(idx)

 def initializer():
      global tree
      tree = dict({"1":[],"2":[],"3":[]})
 data=[1,2,3,1,3,1]
 with Pool(processes=3, initializer=initializer, initargs=()) as pool:
     pool.map(mapTo,data)```

Answer 1

You need to use managed lists if you want the changes to be reflected. So, the following works for me:

from multiprocessing import *
def mapTo(d,tree):
        for idx, item in enumerate(list(d), start=1):
            tree[str(item)].append(idx)

if __name__ == '__main__':
    data=[1,2,3,1,3,1]

    with Pool(processes=3) as pool:
        manager = Manager()
        sharedtree= manager.dict({"1":manager.list(), "2":manager.list(),"3":manager.list()})
        pool.starmap(mapTo, [(data,sharedtree ) for _ in range(3)])

    print({k:list(v) for k,v in sharedtree.items()})

This is the ouput:

{'1': [1, 1, 1, 4, 4, 4, 6, 6, 6], '2': [2, 2, 2], '3': [3, 3, 5, 3, 5, 5]}

Note, you should always use the if __name__ == '__main__': guard when using multiprocessing, also, avoid starred imports...

Edit

You have to do this re-assignment if you are on Python < 3.6, so use this for mapTo:

def mapTo(d,tree):
        for idx, item in enumerate(list(d), start=1):
            l = tree[str(item)]
            l.append(idx)
            tree[str(item)] = l

And finally, you aren't using starmap/map correctly, you are passing the data three times, so of course, everything gets counted three times. A mapping operation should work on each individual element of the data you are mapping over, so you want something like:

from functools import partial
from multiprocessing import *
def mapTo(i_d,tree):
    idx,item = i_d
    l = tree[str(item)]
    l.append(idx)
    tree[str(item)] = l

if __name__ == '__main__':
    data=[1,2,3,1,3,1]

    with Pool(processes=3) as pool:
        manager = Manager()
        sharedtree= manager.dict({"1":manager.list(), "2":manager.list(),"3":manager.list()})
        pool.map(partial(mapTo, tree=sharedtree), list(enumerate(data, start=1)))

    print({k:list(v) for k,v in sharedtree.items()})