gh-153862: Fix curses window.inch() for non-ASCII characters on a wide build (GH-153863)

On a wide build, winch() returns the low 8 bits of the character's code
point with no locale conversion, so inch()/mvinch() disagreed with instr()
for a non-ASCII character of an 8-bit locale ('€' under ISO-8859-15 gave
0xAC instead of 0xA4).  Re-encode the cell to its locale byte via wctob(),
as ncurses does for getbkgd().

(cherry picked from commit a83aaa9)

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

Author: serhiy-storchaka

Doc/library/curses.rst

@@ -1079,6 +1079,8 @@ Window objects
    The bottom 8 bits are the character proper and the upper bits are the attributes;
    extract them with the :data:`A_CHARTEXT` and :data:`A_ATTRIBUTES` bit-masks,
    and the color pair with :func:`pair_number`.
+   The character byte is the locale-encoded byte of the cell's character,
+   consistent with :meth:`instr`.
 
 
 .. method:: window.insch(ch[, attr])

Lib/test/test_curses.py

@@ -509,10 +509,9 @@ def test_read_from_window(self):
         self.assertRaises(ValueError, stdscr.instr, -2)
         self.assertRaises(ValueError, stdscr.instr, 0, 2, -2)
         # A non-ASCII character of an 8-bit locale reads back as its encoded
-        # byte (see _encodable for the set).  instr() returns the locale bytes
-        # for any single-byte character; inch() packs the text into a chtype, so
-        # on a wide build it only round-trips a Latin-1 codepoint (byte ==
-        # codepoint).
+        # byte (see _encodable for the set).  Both instr() and inch() return the
+        # locale byte for any character that fits the locale's single-byte
+        # encoding.
         encoding = stdscr.encoding
         for ch in ('A', 'é', '¤', '€', 'є'):
             try:
@@ -524,8 +523,7 @@ def test_read_from_window(self):
             with self.subTest(ch=ch):
                 stdscr.addstr(2, 0, ch)
                 self.assertEqual(stdscr.instr(2, 0, 1), b)
-                if ord(ch) < 0x100:
-                    self.assertEqual(stdscr.inch(2, 0) & curses.A_CHARTEXT, b[0])
+                self.assertEqual(stdscr.inch(2, 0) & curses.A_CHARTEXT, b[0])
 
     def test_coordinate_errors(self):
         # Addressing a cell outside the window raises curses.error.

Misc/NEWS.d/next/Library/2026-07-17-20-31-00.gh-issue-153862.X3kjoY.rst

@@ -0,0 +1,3 @@
+On a wide :mod:`curses` build, :meth:`curses.window.inch` now returns the
+locale-encoded byte of a non-ASCII character, matching
+:meth:`~curses.window.instr`, instead of the low byte of its code point.

Modules/_cursesmodule.c

@@ -2076,6 +2076,34 @@ _curses_window_insch_impl(PyCursesWindowObject *self, int group_left_1,
     return curses_window_check_err(self, rtn, funcname, "insch");
 }
 
+#ifdef HAVE_NCURSESW
+/* winch() returns the low 8 bits of the character's code point with no locale
+   conversion, unlike instr(), so recover the locale byte from the wide cell
+   when the character maps to exactly one byte, keeping the attribute and color
+   bits in RTN.  A character with no single-byte form is left to winch(). */
+static chtype
+curses_cell_locale_byte(chtype rtn, const cchar_t *cell)
+{
+    wchar_t wstr[CCHARW_MAX + 1];
+    attr_t attrs;
+    short pair;
+    /* getcchar() is not guaranteed to write the text of an empty cell. */
+    wstr[0] = L'\0';
+    if (getcchar(cell, wstr, &attrs, &pair, NULL) == ERR
+        || wstr[0] == L'\0' || wstr[1] != L'\0')
+    {
+        return rtn;
+    }
+    /* wctob() mirrors ncurses' own _nc_to_char(): the single-byte form, or EOF
+       when the character has none in this locale. */
+    int byte = wctob(wstr[0]);
+    if (byte != EOF) {
+        rtn = (rtn & ~(chtype)A_CHARTEXT) | (unsigned char)byte;
+    }
+    return rtn;
+}
+#endif
+
 /*[clinic input]
 _curses.window.inch
 
@@ -2113,6 +2141,14 @@ _curses_window_inch_impl(PyCursesWindowObject *self, int group_right_1,
         curses_window_set_error(self, funcname, "inch");
         return NULL;
     }
+#ifdef HAVE_NCURSESW
+    cchar_t cell = {0};
+    if ((group_right_1 ? mvwin_wch(self->win, y, x, &cell)
+                       : win_wch(self->win, &cell)) != ERR)
+    {
+        rtn = curses_cell_locale_byte(rtn, &cell);
+    }
+#endif
     return PyLong_FromUnsignedLong(rtn);
 }