Python: How combine returned value from 2 functions and appending them into a list using thread?

Question

I am new to thread, I had encountered abnormal result while printing the value inside a list using thread to allow 2 functions to working at the same time and appending the result to a list. Below my code:

import threading
def func1():

  return "HTML"

def func2():

  return "IS FUN"

threadslist = []

thread1 = threading.Thread(target=func1)
thread2 = threading.Thread(target=func2)

x = thread1
y = thread2

x.start()
y.start()

threadslist.append(x)
threadslist.append(y)

print(threadslist)

And here is the result for the list:

[<Thread(Thread-1, stopped 1076)>, <Thread(Thread-2, stopped 7948)>]

Why it storing the Threads object instead of storing ['HTML', 'IS FUN'] ?

The thread isn't actually capturing the return value from each function, just executing it. — APorter1031, Nov 15 '17 at 14:36
You probably want to use a thread pool as seen here: https://stackoverflow.com/questions/6893968/how-to-get-the-return-value-from-a-thread-in-python#14299004 — APorter1031, Nov 15 '17 at 14:37
Possible duplicate of [how to get the return value from a thread in python?](https://stackoverflow.com/questions/6893968/how-to-get-the-return-value-from-a-thread-in-python) — internet_user, Nov 15 '17 at 14:38

score 2 · Answer 1 · answered Nov 15 '17 at 14:41

import threading
threading_list = []
def func1():
   threading_list.append("HTML")

def func2():
   threading_list.append("IS FUN")

thread1 = threading.Thread(target=func1)
thread2 = threading.Thread(target=func2)

x = thread1
y = thread2

x.start()
y.start()

print(threading_list)

Tals · Accepted Answer · 2017-11-15T15:20:05.447

In your threadlist you are saving the Thread variables, so that is what you're seeing in your output is their representation as strings.

You can't get the return value of a function running in a different thread like that. To do what you can either:

Use the multithreading module:

:

from multiprocessing.pool import ThreadPool
def func1():
  return 'HTML'

def func2():
  return 'IS FUN'


pool = ThreadPool(processes=1)

return_values = []

return_values.append(pool.apply(func1, ())) # Using apply for synchronous call directly returns the function return value.
func2_result = pool.applyasync(func2) # Using applyasync for asynchronous call will require a later call.

return_values.append(func2_result.get())  # get return value from asynchronous call to func2.

print(return_values)

Use mutable object, like a list, to save the return values:

:

return_values = []
def func1():
   return_values.append('HTML')

def func2():
   return_values.append('IS FUN')

# rest of your code here
print(return_values)

And you'll get:

['HTML', 'IS FUN']

Hi thank you for the solution, but for solution 1, can I use 1 pool instead of 2 pools? Because the solution got 2 pool = pool1 and pool2, thank you so much — TYL, Nov 15 '17 at 15:03
Yes! And it's much more Pythonic. I updated my answer (Along with an `apply` example :) — Tals, Nov 15 '17 at 15:18

Python: How combine returned value from 2 functions and appending them into a list using thread?

2 Answers2