How to Raise KeyboardInterrupt When Inside Curses

Question

Using the curses module on Windows (via this PDCurses), I am trying to break on a KeyboardInterrupt exception, but it doesn't raise when I press ctrl+c.

Some distilled code:

from curses import wrapper
items = ['a', 'very', 'long', 'list', 'of', 'strings']

def main(screen):
    for item in items:
        screen.addstr(0, 0, item)
        screen.getch()
        screen.refresh()

wrapper(main)

The items list is very long, and right now I can't stop execution half-way. I have to just press keys repeatedly until I get to the end. Heaven forbid I ever try this in a while True:!

When I press ctrl+c, no exception is raised. It does pass to my getch() as 3. Is SOP to raise manually when getch receives 3, or is there a more proper way to avoid swallowing KeyboardInterrupt?

Answer 1

By default curses use raw mode, which turns off interrupt/quit/suspend etc. from the documentation

In raw mode, normal line buffering and processing of interrupt, quit, suspend, and flow control keys are turned off; characters are presented to curses input functions one by one

From the C's curses documentation:

The difference between these two functions (raw and cbreak) is in the way control characters like suspend (CTRL-Z), interrupt and quit (CTRL-C) are passed to the program. In the raw() mode these characters are directly passed to the program without generating a signal.

Since python raises a KeyboardInterrupt when a SIGINT is sent, the fact that it isn't raised is expected. The 3 that you see does represent an interrupt.

Since this is something handled by the C library there is no way to avoid this "swallowing" of the exception. You can however use a simple wrapper for getch that checks when it returns 3 and raises an error accordingly.

Answer 2

This question was asked a long time ago but I ran into the exact same problem. I want a Python program using curses to run on both Windows and Linux. KeyboardInterrupt works exactly as expected on Linux but not Windows. I tried all the curses setup functions but could never get a Ctrl+C to break in to an execution.

The code below seems to work but is not ideal. I can't find a better method so far. The problem with this approach on Windows is that it does not interrupt. The code will do whatever work it's doing in the current loop iteration before it checks for input. (It still works perfectly on Linux.)

import curses
import time

def Main(stdscr):
    stdscr.addstr(0, 0, "Main starting.  Ctrl+C to exit.")
    stdscr.refresh()
    try:
        i = 0
        while True:
            i = i + 1
            stdscr.addstr(1, 0, "Do work in loop. i=" + str(i))
            stdscr.refresh()
            time.sleep(1)

            stdscr.nodelay(1) # Don't block waiting for input.
            c = stdscr.getch() # Get char from input.  If none is available, will return -1.
            if c == 3:
                stdscr.addstr(2, 0, "getch() got Ctrl+C")
                stdscr.refresh()
                raise KeyboardInterrupt
            else:
                curses.flushinp() # Clear out buffer.  We only care about Ctrl+C.
        
    except KeyboardInterrupt:
        stdscr.addstr(3, 0, "Ctrl+C detected, Program Stopping")
        stdscr.refresh()
    
    finally:
        stdscr.addstr(4, 0, "Program cleanup")
        stdscr.refresh()
        time.sleep(3) # This delay just so we can see final screen output

curses.wrapper(Main)

Output on Linux:

Main starting.  Ctrl+C to exit.
Do work in loop. i=4

Ctrl+C detected, Program Stopping
Program cleanup

Output on Windows:

Main starting.  Ctrl+C to exit.
Do work in loop. i=6
getch() got Ctrl+C
Ctrl+C detected, Program Stopping
Program cleanup

Answer 3

Use UGETCHAR_ (implemented below) instead of getch

def UGETCHAR_(scr):
    import curses
    h = scr.getch()
    if h == 3:
        raise KeyboardInterrupt
    if h == 26:
        raise EOFError
    return h

Let me explain.

So first when that function is called, it imports curses [import curses].

Next, it runs the getch() that you are using and puts the result in a variable called h. [h = scr.getch()]

Then, it raises KeyboardInterrupt [raise KeyboardInterrupt] if h is 3 (^C) [if h == 3:] and EOFError [raise KeyboardInterrupt] if h is 26 (^Z) [if h == 26:].

Last, it returns the value of h [return h].