Thanks for this thread — it helped me a lot too.
I learned something about .join() today.
These threads run in parallel:
d.start()
t.start()
d.join()
t.join()
and these run sequentially (not what I wanted):
d.start()
d.join()
t.start()
t.join()
In particular, I was trying to clever and tidy:
class Kiki(threading.Thread):
def __init__(self, time):
super(Kiki, self).__init__()
self.time = time
self.start()
self.join()
This works! But it runs sequentially. I can put the self.start() in __ init __, but not the self.join(). That has to be done after every thread has been started.
join() is what causes the main thread to wait for your thread to finish. Otherwise, your thread runs all by itself.
So one way to think of join() as a “hold” on the main thread — it sort of de-threads your thread and executes sequentially in the main thread, before the main thread can continue. It assures that your thread is complete before the main thread moves forward. Note that this means it’s ok if your thread is already finished before you call the join() — the main thread is simply released immediately when join() is called.
In fact, it just now occurs to me that the main thread waits at d.join() until thread d finishes before it moves on to t.join().
In fact, to be very clear, consider this code:
import threading
import time
class Kiki(threading.Thread):
def __init__(self, time):
super(Kiki, self).__init__()
self.time = time
self.start()
def run(self):
print self.time, " seconds start!"
for i in range(0,self.time):
time.sleep(1)
print "1 sec of ", self.time
print self.time, " seconds finished!"
t1 = Kiki(3)
t2 = Kiki(2)
t3 = Kiki(1)
t1.join()
print "t1.join() finished"
t2.join()
print "t2.join() finished"
t3.join()
print "t3.join() finished"
It produces this output (note how the print statements are threaded into each other.)
$ python test_thread.py
32 seconds start! seconds start!1
seconds start!
1 sec of 1
1 sec of 1 seconds finished!
21 sec of
3
1 sec of 3
1 sec of 2
2 seconds finished!
1 sec of 3
3 seconds finished!
t1.join() finished
t2.join() finished
t3.join() finished
$
The t1.join() is holding up the main thread. All three threads complete before the t1.join() finishes and the main thread moves on to execute the print then t2.join() then print then t3.join() then print.
Corrections welcome. I’m also new to threading.
(Note: in case you’re interested, I’m writing code for a DrinkBot, and I need threading to run the ingredient pumps concurrently rather than sequentially — less time to wait for each drink.)
