CODEWITHSUNDEEP
pythonmultithreadinglist
alvas
alvas

Reputation: 122052

Concurrently run two functions that take parameters and return lists?

I understand that two functions can run in parallel using multiprocessing or threading modules, e.g. Make 2 functions run at the same time and Python multiprocessing for parallel processes.

But the above examples only use print function. Is it possible to run functions that return a list in parallel in python, if so, how?

I've tried with threading:

from threading import Thread
def func1(x):
    return [i*i for i in x]

def func2(x):
    return [i*i*i for i in x]

nums = [1,2,3,4,5]

p1 = Thread(target = func1(nums)).start()
p2 = Thread(target = func2(nums)).start()

print p1
print p2

but i got the follow error:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 808, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 761, in run
    self.__target(*self.__args, **self.__kwargs)
TypeError: 'list' object is not callable

None
None
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 808, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 761, in run
    self.__target(*self.__args, **self.__kwargs)
TypeError: 'list' object is not callable

I've tried inputing args parameter as a tuple, instead of a variable:

import threading
from threading import Thread

def func1(x):
    return [i*i for i in x]

def func2(x):
    return [i*i*i for i in x]

nums = [1,2,3,4,5]

p1 = Thread(target = func1, args=(nums,)).start()
p2 = Thread(target = func2, args=(nums,)).start()

print p1, p2

but it only returns None None, the desired output should be:

[out]:

[1, 4, 9, 16, 25] [1, 8, 27, 64, 125]

Upvotes: 3

Views: 4237

Answers (2)

Velimir Mlaker
Velimir Mlaker

Reputation: 10965

Thread's target function cannot return a value. Or, I should say, the return value is ignored and as such, not communicated back to spawning thread. But here's a couple things you can do:

  1. Communicate back to spawning thread using queue.Queue. Note the wrapper around the original functions:

     from threading import Thread
 from queue import Queue

 def func1(x):
     return [i*i for i in x]

 def func2(x):
     return [i*i*i for i in x]

 nums = [1,2,3,4,5]

 def wrapper(func, arg, queue):
     queue.put(func(arg))

 q1, q2 = Queue(), Queue()
 Thread(target=wrapper, args=(func1, nums, q1)).start() 
 Thread(target=wrapper, args=(func2, nums, q2)).start() 

 print(q1.get(), q2.get())

  2. Use global to access result lists in your threads, as well as the spawning process:

     from threading import Thread

 list1, list2 = [], []

 def func1(x):
     global list1
     list1 = [i*i for i in x]

 def func2(x):
     global list2
     list2 = [i*i*i for i in x]

 nums = [1,2,3,4,5]

 Thread(target = func1, args=(nums,)).start()
 Thread(target = func2, args=(nums,)).start()

 print(list1, list2)

Upvotes: 7

Javier
Javier

Reputation: 1087

Target should only receive the function name. The parameters should be passed with the parameter "args". I cannot paste the code because I am answering from my mobile phone... ;-)

Upvotes: 0

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