How to update global dictionary in a function

Question

I am creating a program like this.

from __future__ import print_function
import multiprocessing

dict1 = dict( (k,v) for (k,v) in zip(range(0,11),range(50,61)))
dict2={}
dict3={}

def fun1(dct):
    #How can I process `dct` as `multiprocessing.Pool` would act like a loop sending chunks of iterable? 
    #I can do:
    #     for i in dct:
    #          dict2.update({dct[i]:i})
    # but `multiprocessing.Pool` will do the looping part automatically. In this case what should be done to index `dct`?

    #dict2.update({dct[i]:i})
    return dict2    

if __name__ == '__main__':
    p=multiprocessing.Pool(2)
    dict3=p.map(fun1,dict1)
    p.close()
    p.join()

    print(dict3) #write in file

I want to modify global variable dict2 inside function fun1 and return the updated global variable to main function to print (write it to a file). However, before that, I am getting error TypeError: 'int' object is unsubscriptable. How can I get this to work?

I read some questions on how to modify a global variable using keyword global, but placing global dict2 in fun1() will reset the variable every time.

Update:

How can I process dct like dct[0] as multiprocessing.Pool would act like a loop sending chunks of iterable?

Answer 1

This gets you your final version of dict2, but I'm not sure that multiprocessing buys you anything in terms of performance.

Warning: Python3 alert

import multiprocessing

dict1 = dict( (k,v) for (k,v) in zip(range(0,11),range(50,61)))
dict2={}

def fun1(dct):
    key, value, dict2 = dct #unpack tuple
    # This code would not update dict2
    # dict2.update({value:key})

    # return the value and key reversed. I assume this is what you are after.
    return {value:key}

if __name__ == '__main__':
    p=multiprocessing.Pool(2)

    # pass the tuple of (key,value,dict2) into each call of fun1
    dict3=p.map(fun1,((key,value,dict2) for key, value in dict1.items()))
    p.close()
    p.join()

    print(dict1)  # original dict
    print(dict2)  # remains empty
    print(dict3)  # this is a list of the results

    for d in dict3:
        dict2.update(d)
    # Now dict2 is populated
    print(dict2)

Output:

{0: 50, 1: 51, 2: 52, 3: 53, 4: 54, 5: 55, 6: 56, 7: 57, 8: 58, 9: 59, 10: 60}
{}
[{50: 0}, {51: 1}, {52: 2}, {53: 3}, {54: 4}, {55: 5}, {56: 6}, {57: 7}, {58: 8}, {59: 9}, {60: 10}]
{50: 0, 51: 1, 52: 2, 53: 3, 54: 4, 55: 5, 56: 6, 57: 7, 58: 8, 59: 9, 60: 10}