Python event loop -- multithreading -- How to run two bits of code simultaneously?

Question

So I'm trying to utilize msvcrt.getch() to make an option to quit(without using KeyBoardInterrupt) anywhere in the program.

My code currently looks like this:

import msvcrt import sys

print("Press q at any time to quit")

while True:
    pressedKey = msvcrt.getch()
    if pressedKey == 'q':    
       sys.exit()
    else:
       # do some setup
       if myvar == "string":
           try:
               # do stuff
           except:
               # do stuff 
       else:
           #do stuff

How do I run the while loop to detect the keypress of q at the same time as I'm running the other (the # do stuff blocks)?

That way, if the user goes ahead with the program, they it'll only run it once. But if they hit q, then the program will quit.

msvcrt.getch() will block if no keys have been pressed. Use msvcrt.kbhit() as I mention in my answer to another of your questions -- it doesn't block. — martineau
– martineau, Commented Mar 13, 2014 at 0:27

Community · Accepted Answer · 2017-05-23 12:21:29Z

1

You could read keys in a separate thread or (better) use msvcrt.kbhit() as @martineau suggested:

#!/usr/bin/env python
import msvcrt
from Queue import Empty, Queue
from threading import Thread

def read_keys(queue):
    for key in iter(msvcrt.getch, 'q'): # until `q`
        queue.put(key)
    queue.put(None) # signal the end

q = Queue()
t = Thread(target=read_keys, args=[q])
t.daemon = True # die if the program exits
t.start()

while True:
    try:
        key = q.get_nowait() # doesn't block
    except Empty:
        key = Empty
    else:
        if key is None: # end
            break
    # do stuff

If I wanted to do something in the main code when the second thread detected a certain keypress, how would I act on that?

You do not react to the key press in the main thread until code reaches q.get_nowait() again i.e., you won't notice the key press until "do stuff" finishes the current iteration of the loop. If you need to do something that may take a long time then you might need to run it in yet another thread (start new thread or use a thread pool if blocking at some point is acceptable).

edited May 23, 2017 at 12:21

CommunityBot

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answered Mar 13, 2014 at 0:05

jfs

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1 Comment

evamvid Over a year ago

Also, how do I switch threads? i.e, If I wanted to do something in the main code when the second thread detected a certain keypress, how would I act on that?

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