RuntimeError: can't re-enter readline

Question

I am making a Python 3 app that has a prompt where the user enters commands. I'm running Python 3.5 on macOS Sierra.

I'm trying to prevent exiting with Ctrl+C and having users exit instead by simply typing exit into the prompt.

I am doing this using this method: https://stackoverflow.com/a/6019460/7450368

However, when prompting the user again, I get RuntimeError: can't re-enter readline.

Is there a way I can fix this?

Here's my code:

import interpret
import signal
def SigIntHand(SIG, FRM):
    print("\nUse 'exit' to exit.")
    prompt()

signal.signal(signal.SIGINT, SigIntHand)




version = ("v0.0.3")




class CommandError(Exception):
    pass




def prompt():
    try:
        task = input("Einstein " + version + "  > ")
    except RuntimeError:
        task = input("Einstein " + version + "  > ")
    try:
        execute(task)
    except CommandError as e:
        print(e)
    prompt()


def execute(command):
    command = command.lower()
    interpret.interpret(command)

prompt()

interpret.interpret("command") is called by main.py to execute the command command if it exists.

Any help would be hugely appreciated. Thanks!

What are you expecting to throw the runtime error in the first try-except of `prompt`? — Christopher Apple, Apr 13 '17 at 20:30
I don't expect anything to cause it, but it happens nonetheless. I just get `RuntimeError: can't re-enter readline`. — , Apr 14 '17 at 13:36
The user pressing `Ctrl+C` triggers it, but I'm not sure why. Adding a `try`...`except` allows it to be caught once, but if the user presses `Ctrl+C` again, the same error pops up. — , Apr 14 '17 at 13:38
I can't reproduce by replacing `interpret.interpret` with `os.system` which is sort of what I was expecting you to be doing, could you try to post a [MCVE](http://stackoverflow.com/help/mcve)? — Tadhg McDonald-Jensen, Apr 14 '17 at 13:54

score 0 · Answer 1 · answered Apr 14 '17 at 15:25

Your control flow looks a bit strange. To handle the user pressing Ctrl+C more than once, you should use a loop to keep asking for input.

Also, you can catch a KeyboardInterrupt in Python which gets raised every time you hit Ctrl+C. It's a bit easier than messing with signal handlers.

version = ("v0.0.3")

class CommandError(Exception):
    pass

def prompt():
    try:
        return input("Einstein " + version + "  > ")
    except KeyboardInterrupt:
        print('') # Go to the next line so printing looks better.
        return None

def force_prompt():
    task = None
    while task is None:
        task = prompt()
    return task

def get_and_execute_command():
    task = force_prompt()
    try:
        execute(task)
    except CommandError as e:
        print(e)

def execute(command):
    command = command.lower()
    print('Command: %s' % command)

get_and_execute_command()

The reason I don't catch `KeyboardInterrupt` is that (for me at least) it doesn't prevent the user exiting with `Ctrl+C` - I can `print` out 'Exit with 'exit' next time!` but it still exits after that. — , Apr 15 '17 at 06:56

RuntimeError: can't re-enter readline

1 Answers1