How to get the return value from a thread using python

0 votes

How to get the value 'foo' which is returned from the thread as shown below?

from threading import Thread

def foo(bar):
    print 'hello {}'.format(bar)
    return 'foo'

thread = Thread(target=foo, args=('world!',))
thread.start()
ret = thread.join()
print ret

 Returns None in the above case.

Thanks for the help!

Dec 3, 2018 in Python by Anirudh
• 2,080 points

edited Dec 19, 2018 by Anirudh 53,023 views

3 answers to this question.

0 votes

You don't need to change your existing code:

import Queue
from threading import Thread

def foo(bar):
    print 'hello {0}'.format(bar)
    return 'foo'

que = Queue.Queue()

t = Thread(target=lambda q, arg1: q.put(foo(arg1)), args=(que, 'world!'))
t.start()
t.join()
result = que.get()
print result

It can be also easily adjusted to a multi-threaded environment as well. Check this below:

import Queue
from threading import Thread

def foo(bar):
    print 'hello {0}'.format(bar)
    return 'foo'

que = Queue.Queue()
threads_list = list()

t = Thread(target=lambda q, arg1: q.put(foo(arg1)), args=(que, 'world!'))
t.start()
threads_list.append(t)

# Add more threads here
...
threads_list.append(t2)
...
threads_list.append(t3)
...

# Join all the threads
for t in threads_list:
    t.join()

# Check thread's return value
while not que.empty():
    result = que.get()
    print result

Hope this helps, happy learning!

answered Dec 3, 2018 by Nymeria
• 3,520 points

edited Dec 19, 2018 by Nymeria

What does 

lambda q, 

Do?

Hey @Ali,

Lambda function always declared with no name but which can have any number of arguments but one expression. Here 'lambda q' is the function where 'q' is the argument and will get evaluated and returned.

Thanks a lot...Your answer is just awesome :)
Very beautiful answer that helps to understand.

I want to return something when the thread is alive. How can I do that?

Thanks in advance

Hi, @Shail,

Thank you for your contribution to the Edureka community, but whatever you want to return, you can return here.

0 votes

In Python 3.2+, stdlib concurrent. futures module provides a higher-level API to threading, including passing return values or exceptions from a worker thread back to the main thread:

import concurrent.futures

def foo(bar):
    print('hello {}'.format(bar))
    return 'foo'

with concurrent.futures.ThreadPoolExecutor() as executor:
    future = executor.submit(foo, 'world!')
    return_value = future.result()
    print(return_value)
answered Dec 15, 2020 by Gitika
• 65,870 points
0 votes

FWIW, the multiprocessing module has a nice interface for this using the Pool class. And if you want to stick with threads rather than processes, you can just use the multiprocessing.pool.ThreadPool class as a drop-in replacement.

def foo(bar, baz):
  print 'hello {0}'.format(bar)
  return 'foo' + baz

from multiprocessing.pool import ThreadPool
pool = ThreadPool(processes=1)

async_result = pool.apply_async(foo, ('world', 'foo')) # tuple of args for foo

# do some other stuff in the main process

return_val = async_result.get()  # get the return value from your function.
answered Dec 15, 2020 by Roshni
• 10,420 points

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