Reading input from raw_input() without having the prompt overwritten by other threads in Python

Question

I'm trying to let the user input commands at a console using raw_input(), this works fine. The problem is I have background threads that occasionally output log-information to the screen and when they do they mess up the input prompt (since the output go wherever the cursor happens to be at the moment).

This is a small Python program that illustrate what i mean.

#!/usr/bin/env python
import threading
import time

def message_loop():
    while True:
        time.sleep(1)
        print "Hello World"

thread = threading.Thread(target = message_loop)
thread.start()

while True:
    input = raw_input("Prompt> ")
    print "You typed", input

This is an example of what it could look like when I run it:

Prompt> Hello World
Hello World
Hello World
Hello World
test
You typed test
Prompt> Hello World
Hello World
Hello World
hellHello World
o
You typed hello
Prompt> Hello World
Hello World
Hello World
Hello World

What I want is for the prompt to move along with the output from the thread. Like so:

Hello World
Hello World
Prompt> test
You typed test
Hello World
Hello World
Hello World
Hello World
Hello World
Prompt> hello
You typed hello
Hello World
Hello World
Hello World
Hello World
Prompt> 

Any ideas on how to achieve this without resorting to ugly hacks? :)

Answer 1

I don't think it's possible. How should that behave anyways? Nothing shows up until the user presses Enter? If that's so, output would only come when the user issues a command (or whatever your system expects), and that doesn't sound desirable.

Methinks your threads should output to another file.

Answer 2

I think you need something that lets you dynamically print/delete/overwrite text from the terminal window e.g. how the UNIX watch or top commands work.

I think in your case you would print "Prompt>" but then when you get a "Hello World" you overwrite "Prompt>" with "Hello World", and then print "Prompt>" on the line below. I don't think you can do that with regular output printing to the terminal.

You might be able to do what you want using Python's curses library. I have never used it so I can't tell you how to solve your problem (or if the module will even be able to solve your problem), but I think it is worth taking a look into. A search for "python curses tutorial" provided a PDF tutorial document which seems helpful.

Answer 3

you need to update stdout from a single thread, not from multiple threads... or else you have no control over interleaved i/o.

you will want to create a single thread for output writing.

you can use a Queue in the thread and have all other threads write their output logging info to it.. then read from this Queue and write to stdout at appropriate times along with your prompt message.