Show prompt when waiting for non-blocking user input in Python

Question

I have a non-blocking read from sys.stdin using select which alternates handlers for my socket and user input:

while True:
    input_ready, _, _ = select.select([my_socket, sys.stdin], [], [])
    for sender in input_ready:
        if sender == sys.stdin:
            process_user_input()
        elif sender == my_socket:
            process_socket_reply()

in process_user_input() I have:

command = input()

to get and handle user input.

I want to show a prompt to give a terminal-like view to my users; just like what when using input('> ') in a blocking stdin input we can achieve.

But, if I use input('> ') in my process_user_input(), > will be printed after user entered his command (as expected!)

some_command
> Invalid command.

How can I do that?

NOTE: As I have another events happening in my code, there may be some prints in stdout (thus, print('> ', end='') will fail). So I want my > to be present whenever I'm waiting for inputs, even after some prints in terminal.

Jeff Booth · Accepted Answer · 2019-06-04 06:52:23Z

1

When you write output to a terminal, it generally won't appear on the terminal immediately. Instead, it waits in a buffer somewhere, to be "flushed" to the terminal display at some opportune time. On most systems, that time is when your code writes a newline.

Since you want the '> ' prompt to appear on the same line as your user's input, you can't write a newline after it to flush the buffer. Instead, you must add the flush argument to your print function call:

print(end='> ', flush=True)

If you want the prompt to appear after other output is printed on the screen, you could:

Add print(end='> ', flush=True) to the end of any other code that produces output while your program waits for user input.
Create your own implementation of Python's standard output stream sys.stdout, which would add print(end='> ', flush=True) whenever other parts of your program print a newline.
Use an advanced terminal toolkit such as Python Prompt Toolkit or Curses to always keep your '> ' prompt at the bottom of the terminal window.

answered Jun 4, 2019 at 6:52

Jeff Booth

4992 silver badges6 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Ali Tou Over a year ago

Sorry for my very late reply. Actually I read your reply on time and used your first solution. But other solutions you proposed are interesting to me and I wish I can make my hands dirty with them later. Thanks.

Collectives™ on Stack Overflow

Show prompt when waiting for non-blocking user input in Python

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related