`set_term()` does not refresh `curses.LINES`, `COLS` or `COLORS`

[fedonman]

Bug description:

curses.LINES, curses.COLS, curses.COLORS and curses.COLOR_PAIRS are copied into the module namespace when a terminal is initialized, and set_term() does not refresh them, so after a switch they describe a different terminal. curses.newwin(curses.LINES, curses.COLS) then sizes a window for the wrong screen, and a colour checked against curses.COLORS is rejected by init_pair(). screen.use() behaves the same way.

import curses, fcntl, os, struct, termios

def screen(kind, rows, cols):
    slave = os.openpty()[1]
    fcntl.ioctl(slave, termios.TIOCSWINSZ, struct.pack('HHHH', rows, cols, 0, 0))
    return curses.newterm(kind, slave, slave)

small = screen('xterm', 18, 60)              # 8 colours
curses.start_color()
big = screen('xterm-256color', 32, 120)      # 256 colours
curses.start_color()
curses.set_term(small)

print('current screen', small.stdscr.getmaxyx(), curses.ncurses_version)
print('LINES, COLS   ', (curses.LINES, curses.COLS))
print('COLORS        ', curses.COLORS)
try:
    curses.init_pair(1, curses.COLORS - 1, 0)
except ValueError as exc:
    print('init_pair     ', exc)

$ ./python repro.py
current screen (18, 60) curses.ncurses_version(major=6, minor=6, patch=20251231)
LINES, COLS    (32, 120)
COLORS         256
init_pair      Color number is greater than COLORS-1 (7).

Expected: (18, 60) and 8, the values of the screen that is current. Doc/library/curses.rst calls LINES "the height of the screen" and COLORS "the maximum number of colors the terminal can support", with nothing about them following one screen only.

set_term() does call the internal update_lines_cols(), but that helper re-reads the ncurses LINES/COLS globals, which set_term() leaves alone, and nothing copies COLORS/COLOR_PAIRS, whose C value did change.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-155976: Refresh COLORS and COLOR_PAIRS in curses set_term #155994

[pranavchoudhary-tech]

I'd like to take this up. I've updated update_lines_cols in Modules/_cursesmodule.c to refresh COLORS and COLOR_PAIRS when start_color is active, and added a test in Lib/test/test_curses.py. I will open a PR shortly.

[pranavchoudhary-tech]

I've opened PR #155994 with the fix and test coverage.