curses: error when adding a symbol to the bottom right corner of the window

[pikhovkin]

Bug report

Bug description:

import curses

def main(stdscr):
    stdscr.clear()
    # height, width = stdscr.getmaxyx()
    height, width = curses.LINES, curses.COLS
    win = curses.newwin(height, width, 0, 0)
    win.addstr(0, 0, '+')
    win.addstr(0, width - 1, '+')  # width - 1: no error
    # win.addstr(height - 1, width - 2, '+')  # width - 2: no error
    win.addstr(height - 1, width - 1, '+')  # width - 1: error
    # win.addch(height - 1, width - 1, '+')  # width - 1: error
    win.addstr(height - 1, 0, '+')
    win.touchwin()
    win.refresh()
    win.getch()

if __name__ == '__main__':
    curses.update_lines_cols()
    curses.wrapper(main)

Traceback (most recent call last):
  File "/.../demo.py", line 20, in <module>
    curses.wrapper(main)
    ~~~~~~~~~~~~~~^^^^^^
  File "/.../3.13.2/lib/python3.13/curses/__init__.py", line 94, in wrapper
    return func(stdscr, *args, **kwds)
  File "/.../demo.py", line 11, in main
    win.addstr(height - 1, width - 1, '+')  # width - 1: error
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
_curses.error: addwstr() returned ERR

CPython versions tested on:

3.12, 3.13

Operating systems tested on:

Linux

[brianschubert]

Hi! This is a documented behavior that originates from the underlying (n)curses library. From the curses.addch docs:

Writing outside the window, subwindow, or pad raises a curses.error. Attempting to write to the lower right corner of a window, subwindow, or pad will cause an exception to be raised after the character is printed.

More context for this behavior can be found in the the manpage for addch:

At the bottom of the current scrolling region, and if scrollok is enabled, the scrolling region is scrolled up one line.
If scrollok is not enabled, writing a character at the lower right margin succeeds. However, an error is returned because it is not possible to wrap to a new line

(emphasis mine)

In your code, you can work around this by catching curses.error when writing to the bottom-right corner. For example:

with contextlib.suppress(curses.error):
    win.addch(height - 1, width - 1, '+')

[pikhovkin]

@brianschubert thank you for your answer!