Python 2.7: read from stdin without prompting

Question

I'm trying to make an Arduino Yun alarm system. It needs to make requests to my web server to update its stats. It also needs to monitor a button and a motion sensor. The Linux side is running a python script that will make web requests. I need to have the Arduino send its status to the python script. In the python script, I need to read from the Arduino side. I can do that with print raw_input(), but I want it to only read if there is something available, I don't want it to block if nothing is available. For example:

import time
while 1:
    print "test"
    time.sleep(3)
    print raw_input()
    time.sleep(3)

If I run it, I want it to print:

test

(6 seconds later)

test

Instead of

test
(Infinite wait until I type something in)

I've tried threads, but they're a little hard to use.

Can you use sockets? I mean, write some "mini web server" using Python. Script that open socket, and waiting for any connection. — Alexander R., Mar 28 '15 at 18:22
there are many ways you could do this (sockets, telnet, files I/O, queues, ...) but I don't think raw_input() should be it — Julien Spronck, Mar 28 '15 at 18:25
also, sys.stdin is a file object that you can read using `sys.stdin.read()` — Julien Spronck, Mar 28 '15 at 18:26
Related: http://stackoverflow.com/questions/3471461/raw-input-and-timeout — jkd, Mar 28 '15 at 18:27
http://www.thecodingforums.com/threads/timeout-at-command-prompt.353297/ may point you is a profitable direction. — boardrider, Mar 29 '15 at 13:48

score 0 · Answer 1 · answered Mar 28 '15 at 19:15

0

Simple solution which waits for single line of data. Uses file-like sys.stdin object.

import sys

while True:
    print "Pre"
    sys.stdin.readline()
    print "Post"

answered Mar 28 '15 at 19:15

Łukasz Rogalski

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Unfortunately, this won't work as the script is paused as `sys.stdin.readline()` is executed. – lights0123 Mar 29 '15 at 00:05

score 0 · Accepted Answer · answered Mar 29 '15 at 00:19

I looked at jakekimds's comment, and I saw that I could just do:

while 1:
    rlist,_,_=select([sys.stdin],[],[],0)
    content=""
    while rlist:
        content+=raw_input()
        rlist,_,_=select([sys.stdin],[],[],0)
    print "blocking task - content:"+content
    time.sleep(5)

This will:

If content from stdin is available, then store it in content.
Execute a blocking task.
Sleep for 5 seconds.
Go back to step 1.

Python 2.7: read from stdin without prompting

2 Answers2