I'm working on a bot for a competition that receives its input through sys.stdin and uses Python's print() for output. I have the following:

import sys

def main():
    while True:
        line = sys.stdin.readline()
        parts = line.split()
        if len(parts) > 0:
            # do stuff

The problem is that the input comes in through a stream and using the above, blocks me from printing anything back until the stream is closed. What can I do to make this work?

By turning blocking off you can only read a character at a time. So, there is no way to get readline() to work in a non-blocking context. I assume you just want to read key presses to control the robot.

I have had no luck using select.select() on Linux and created a way with tweaking termios settings. So, this is Linux specific but works for me:

import atexit, termios
import sys, os
import time


old_settings=None

def init_any_key():
   global old_settings
   old_settings = termios.tcgetattr(sys.stdin)
   new_settings = termios.tcgetattr(sys.stdin)
   new_settings[3] = new_settings[3] & ~(termios.ECHO | termios.ICANON) # lflags
   new_settings[6][termios.VMIN] = 0  # cc
   new_settings[6][termios.VTIME] = 0 # cc
   termios.tcsetattr(sys.stdin, termios.TCSADRAIN, new_settings)


@atexit.register
def term_any_key():
   global old_settings
   if old_settings:
      termios.tcsetattr(sys.stdin, termios.TCSADRAIN, old_settings)


def any_key():
   ch_set = []
   ch = os.read(sys.stdin.fileno(), 1)
   while ch is not None and len(ch) > 0:
      ch_set.append( ord(ch[0]) )
      ch = os.read(sys.stdin.fileno(), 1)
   return ch_set


init_any_key()
while True:
   key = any_key()
   if key is not None:
      print(key)
   else:
      time.sleep(0.1)

A better Windows or cross-platform answer is here: Non-blocking console input?