gh-153864: Fix curses window.insch() for non-ASCII characters on a wide build

On a wide build, winsch() does not locale-decode a byte above 127, unlike
waddch(), so insch() inserted '¤' (U+00A4) instead of '€' for byte 0xA4
under ISO-8859-15.  Decode the byte with btowc() and insert it as a wide
character, like addch().

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_curses.py

@@ -785,13 +785,10 @@ def test_output_character(self):
                 stdscr.move(2, 0)
                 stdscr.echochar(v)
                 self.assertEqual(self._read_char(2, 0), c)
-                # insch() round-trips a byte only where its code point equals
-                # the byte value (Latin-1): on a wide build ncurses winsch
-                # stores a printable byte directly as a code point instead of
-                # decoding it through the locale.
-                if ord(c) < 0x100:
-                    stdscr.insch(1, 0, v)
-                    self.assertEqual(self._read_char(1, 0), c)
+                # insch() decodes the byte through the locale like addch(), so
+                # it round-trips the same character.
+                stdscr.insch(1, 0, v)
+                self.assertEqual(self._read_char(1, 0), c)
 
         # The same characters supplied as a str.  Unlike the int path above, a
         # str is stored as a wide-character cell on a wide build, so every

Misc/NEWS.d/next/Library/2026-07-17-20-56-32.gh-issue-153864.WmA9By.rst

@@ -0,0 +1,3 @@
+On a wide :mod:`curses` build, :meth:`curses.window.insch` now inserts a
+non-ASCII byte as the character it encodes in the window's encoding,
+consistently with :meth:`~curses.window.addch`, instead of its code point.

Modules/_cursesmodule.c

@@ -3544,6 +3544,24 @@ _curses_window_insch_impl(PyCursesWindowObject *self, int group_left_1,
     if (type == 0) {
         return NULL;
     }
+    if (type == 1) {
+        /* winsch() does not locale-decode a byte above 127 on a wide build,
+           unlike waddch(), so decode it here and insert it as a wide
+           character. */
+        chtype cch = ch_ & A_CHARTEXT;
+        if (cch > 127) {
+            wint_t wc = btowc((int)cch);
+            if (wc != WEOF) {
+                wchar_t wstr[2] = { (wchar_t)wc, L'\0' };
+                attr_t cattr = (attr_t)((ch_ | attr) & ~(chtype)A_CHARTEXT);
+                if (curses_setcchar(&wch, wstr, cattr, PAIR_NUMBER(cattr)) == ERR) {
+                    curses_window_set_error(self, "setcchar", "insch");
+                    return NULL;
+                }
+                type = 2;
+            }
+        }
+    }
     if (type == 2) {
         if (!group_left_1) {
             rtn = wins_wch(self->win, &wch);