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Question: None of my threads seem to run: why?

Answer: As soon as the main thread exits, all threads are killed. Your main thread is running too quickly, giving the threads no time to do any work.

A simple fix is to add a sleep to the end of the program that's long enough for all the threads to finish:

import threading, time

def thread_task(name, n):
for i in range(n): print name, i

for i in range(10):
T = threading.Thread(target=thread_task, args=(str(i), i))
T.start()


time.sleep(10) # <----------------------------!

But now (on many platforms) the threads don't run in parallel, but appear to run sequentially, one at a time! The reason is that the OS thread scheduler doesn't start a new thread until the previous thread is blocked.

A simple fix is to add a tiny sleep to the start of the run function:

def thread_task(name, n):
time.sleep(0.001) # <---------------------!
for i in range(n): print name, i

for i in range(10):
T = threading.Thread(target=thread_task, args=(str(i), i))
T.start()

time.sleep(10)

Instead of trying to guess how long a time.sleep() delay will be enough, it's better to use some kind of semaphore mechanism. One idea is to use the Queue module to create a queue object, let each thread append a token to the queue when it finishes, and let the main thread read as many tokens from the queue as there are threads.

As soon as the main thread exits, all threads are killed. Your main thread is running too quickly, giving the threads no time to do any work.

A simple fix is to add a sleep to the end of the program that's long enough for all the threads to finish:

import threading, time

def thread_task(name, n):
for i in range(n): print name, i

for i in range(10):
T = threading.Thread(target=thread_task, args=(str(i), i))
T.start()


time.sleep(10) # <----------------------------!

But now (on many platforms) the threads don't run in parallel, but appear to run sequentially, one at a time! The reason is that the OS thread scheduler doesn't start a new thread until the previous thread is blocked.

A simple fix is to add a tiny sleep to the start of the run function:

def thread_task(name, n):
time.sleep(0.001) # <---------------------!
for i in range(n): print name, i

for i in range(10):
T = threading.Thread(target=thread_task, args=(str(i), i))
T.start()

time.sleep(10)

Instead of trying to guess how long a time.sleep() delay will be enough, it's better to use some kind of semaphore mechanism. One idea is to use the Queue module to create a queue object, let each thread append a token to the queue when it finishes, and let the main thread read as many tokens from the queue as there are threads. Source: CoolInterview.com

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