How to end a loop with user input python

I have an infinite while loop that I want to break out of when the user presses a key. Usually I use raw_input to get the user's response; however, I need raw_input to not wait for the response. I want something like this:

print 'Press enter to continue.'
while True:
    # Do stuff
    #
    # User pressed enter, break out of loop

This should be a simple, but I can't seem to figure it out. I'm leaning towards a solution using threading, but I would rather not have to do that. How can I accomplish this?

asked Dec 13, 2013 at 22:16

7

You can use non-blocking read from stdin:

import sys
import os
import fcntl
import time

fl = fcntl.fcntl[sys.stdin.fileno[], fcntl.F_GETFL]
fcntl.fcntl[sys.stdin.fileno[], fcntl.F_SETFL, fl | os.O_NONBLOCK]
while True:
    print["Waiting for user input"]
    try:
        stdin = sys.stdin.read[]
        if "\n" in stdin or "\r" in stdin:
            break
    except IOError:
        pass
    time.sleep[1]

answered Dec 13, 2013 at 22:43

1

I think you can do better with msvcrt:

import msvcrt, time
i = 0
while True:
    i = i + 1
    if msvcrt.kbhit[]:
        if msvcrt.getwche[] == '\r':
            break
    time.sleep[0.1]
print[i]

Sadly, still windows-specific.

answered Dec 13, 2013 at 22:49

octrefoctref

6,3636 gold badges27 silver badges42 bronze badges

On python 3.5 you can use the following code. It can be adjusted for a specific keystroke. The while loop will keep running until the user presses a key.

import time
import threading

# set global variable flag
flag = 1

def normal[]:
    global flag
    while flag==1:
        print['normal stuff']
        time.sleep[2]
        if flag==False:
            print['The while loop is now closing']


def get_input[]:
    global flag
    keystrk=input['Press a key \n']
    # thread doesn't continue until key is pressed
    print['You pressed: ', keystrk]
    flag=False
    print['flag is now:', flag]

n=threading.Thread[target=normal]
i=threading.Thread[target=get_input]
n.start[]
i.start[]

answered Nov 28, 2017 at 20:43

gtcodergtcoder

1412 silver badges6 bronze badges

I could not get some of the popular answers working. So I came up with another approach using the CTRL + C to plug in user input and imbibe a keyboard interrupt. A simple solution can be using a try-catch block,

i = 0
try:
    while True:
        i+=1
        print[i]
        sleep[1]
except:
    pass
# do what you want to do after it...

I got this idea from running a number of servers like flask and django. This might be slightly different from what the OP asked, but it might help someone else who wanted a similar thing.

answered Feb 2, 2019 at 5:47

lu5erlu5er

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What you need is a non-blocking raw input, if you don't want to use threads there is a simple solution like this one below where he is doing a timeout of 20 ms and then raise and exception if the user doesn't press a key, if he does then the class returns the key pressed.

import signal

class AlarmException[Exception]:
    pass

def alarmHandler[signum, frame]:
    raise AlarmException

def nonBlockingRawInput[prompt='', timeout=20]:
    signal.signal[signal.SIGALRM, alarmHandler]
    signal.alarm[timeout]
    try:
        text = raw_input[prompt]
        signal.alarm[0]
        return text
    except AlarmException:
        print '\nPrompt timeout. Continuing...'
    signal.signal[signal.SIGALRM, signal.SIG_IGN]
    return ''

Source code

answered Dec 13, 2013 at 22:44

moisesvegamoisesvega

4251 gold badge5 silver badges16 bronze badges

Using the msvcrt module as thebjorn recommended I was able to come up with something that works. The following is a basic example that will exit the loop if any key is pressed, not just enter.

import msvcrt, time
i = 0
while True:
    i = i + 1
    if msvcrt.kbhit[]:
        break
    time.sleep[0.1]
print i

answered Dec 13, 2013 at 22:32

I have defined the function which ask number input from the user and returns the factorial of that number. If user wants to stop they have to press 0 and then it will exit from the loop. We can specify any specific key to input 'n' to exit from the loop.

import math

def factorial_func[n]:

    return math.factorial[n]

while True:
    n = int[input["Please enter the number to find factorial: "]]
    print[factorial_func[n]]
    if n == 0:
       exit[]

answered Jan 26, 2021 at 14:26

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