exit while loop with user prompt in python

Question

I've been browsing a long time searching an answer to this.

I'm using Python 2.7 in Unix.

I have a continuous while loop and I need an option where user could interrupt it, do something and after that the loop would continue.

Like:

while 2 > 1:
     for items in hello:
         if "world" in items:
             print "hello"
         else:
             print "world"

      time.sleep(5)
      here user could interrupt the loop with pressing "u" etc. and modify elements inside he loop.

I started testing out with raw_input, but since it prompts me out every cycle, it's something that I don't need.

I tried methods mentioned here:

Keyboard input with timeout in Python

couple of times, but none of those seem to work how I wish.

How do you wish it to work? I suppose that there's only one word for you then: threads. — freakish
– freakish, Commented Jul 31, 2013 at 14:08
raw_input() won't work fo reading one character, because "ordinary" console input is line buffered. See this recipe for unbuffered reading code.activestate.com/recipes/134892 — WGH
– WGH, Commented Jul 31, 2013 at 14:12
I use True in the original code. Thanks for mentioning threads. I will look into those. — jester112358
– jester112358, Commented Jul 31, 2013 at 14:13
What about the signal approach (discussed in the question you linked to) isn't working for you? It seems like it should be a workable solution... Otherwise, you can try using select (docs.python.org/release/2.5.2/lib/module-select.html) to poll the keyboard and see if it has any new input. — seaotternerd
– seaotternerd, Commented Jul 31, 2013 at 14:16

Steve Barnes · Accepted Answer · 2013-07-31 14:40:10Z

4

>>> try:
...    print 'Ctrl-C to end'
...    while(True):
...       pass
... except KeyboardInterrupt, e:
...    print 'Stopped'
...    raise
...
Ctrl-C to end
Stopped
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
KeyboardInterrupt
>>>

Obviously you need to replace pass with whatever you are doing and print with the aftermath.

answered Jul 31, 2013 at 14:40

Steve Barnes

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Comments

seaotternerd · Accepted Answer · 2013-07-31 15:28:57Z

Here's how to do it by polling stdin:

import select, sys
p = select.poll()
p.register(sys.stdin, 1) #select the file descriptor, 
            #in this case stdin, which you want the 
            #poll object to pay attention to. The 1 is a bit-mask 
            #indicating that we only care about the POLLIN 
            #event, which indicates that input has occurred

while True:
     for items in hello:
         if "world" in items:
              print "hello"
         else:
              print "world"

     result = p.poll(5) #this handles the timeout too
     if len(result) > 0: #see if anything happened
         read_input = sys.stdin.read(1)
         while len(read_input) > 0:
              if read_input == "u":
                  #do stuff!
              read_input = sys.stdin.read(1) #keep going 
                            #until you've read all input

Note: This probably wouldn't work in Windows.

Jeremy Kalas · Accepted Answer · 2013-07-31 14:52:31Z

1

You could do nested while loops, something structured like this:

while true:
    go = True
    while go:
     for items in hello:
         if "u" in items:
             go = False
         else if "world" in items:
             print "hello"
         else:
             print "world"
    #Here you parse input to modify things in the nested loop, include a condition to set       
    #go back to true to reenter the loop

edited Jul 31, 2013 at 14:52

answered Jul 31, 2013 at 14:22

Jeremy Kalas

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3 Comments

user2555451 Over a year ago

Python is a case-sensitive language. "true" and "false" are not defined. They should be "True" and "False".

jester112358 Over a year ago

This iterates through "hello" and searches "u" from there, which was not the problem.

Jeremy Kalas Over a year ago

I know what it does. I assumed hello was his list of input, though there wasn't much documentation. I though your problem was exiting a loop when a command is entered, and reentering it later, which this outlines (to the best of my ability, without understanding what any of your variables do) how to do.

WGH · Accepted Answer · 2013-07-31 14:48:48Z

import sys
import os
import time

import termios
import tty
import fcntl
import errno

KEY = "u"

def read_characters(f):
    fd = f.fileno()

    # put descriptor in non-blocking mode
    flags = fcntl.fcntl(fd, fcntl.F_GETFL)
    fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK)

    try:
        while 1:
            try:
                yield f.read(1)
            except IOError as e:
                if e.errno == errno.EAGAIN:
                    # no more characters to read
                    # right now
                    break
    finally:
        # restore blocking mode
        fcntl.fcntl(fd, fcntl.F_SETFL, flags)

def main():
    fd = sys.stdin.fileno()

    # save current termios settings
    old_settings = termios.tcgetattr(fd)

    # tty sets raw mode using termios module
    tty.setraw(fd)

    try:
        while True:
            time.sleep(1)

            for c in read_characters(sys.stdin):
                if c == KEY: break
            else:
                c = None

            if c == KEY: break

            sys.stdout.write("still going\r\n")

    finally:
        # restore terminal settings
        termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)

if __name__ == "__main__":
    main()

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