Python Threads object append to list

Question

I am learning multi-thread in python.I often see when the program use multi thread,it will append the thread object to one list, just as following:

# imports
import threading
import time

def worker():
    print "worker...."
    time.sleep(30)

threads = []
for i in range(5):
    thread = threading.Thread(target=worker)
    threads.append(thread)
    thread.start()

I think append the thread object to list is good practice, but I don't know why should we do this?

Answer 1

This is common practice. Taking your example:

# imports
import threading
import time

def worker():
    print "worker...."
    time.sleep(30)

threads = []
for i in range(5):
    thread = threading.Thread(target=worker)
    threads.append(thread)
    thread.start()

One might want to wait for every thread to finish its work:

for thread in threads:  # iterates over the threads
    thread.join()       # waits until the thread has finished work

---

Without storing the threads in some data structure you would have to do it (create, start, join, ...) manually:

thread_1 = threading.Thread(target=worker)
(...)
thread_n = threading.Thread(target=worker)

thread_1.start()
(...)
thread_n.start()

thread_1.join()
(...)
thread_n.join()

---

As you see (and can imagine): the more you work with the threads, the more "paperwork" would be created if you handle every thread manually. This fastly gets too much of a hassle. Additionally your code would be more confusing and less maintainable.