gh-155499: Fix curses border() and box() rejecting 0 mixed with str/bytes

Lib/test/test_curses.py

@@ -1521,6 +1521,21 @@ def test_borders_and_lines(self):
                     win.box(v, v)
                     self.assertEqual(win.instr(0, 1, 1), b)
 
+    def test_border_box_zero_sentinel(self):
+        # gh-155499: 0 must still mean "use the default character" even
+        # when mixed with a one-character str/bytes argument.
+        win = curses.newwin(5, 10, 5, 2)
+        win.border('|', '|', '-', '-')
+        win.border('|', '|', '-', '-', 0, 0, 0, 0)
+        win.border(0, '|', 0, '-', 0, 0, 0, 0)
+        self.assertRaises(TypeError, win.border,
+                          '|', '|', '-', '-', 1, 0, 0, 0)
+        win.box('|', 0)
+        win.box(0, '-')
+        win.box(0, 0)
+        self.assertRaises(TypeError, win.box, '|', 1)
+        self.assertRaises(TypeError, win.box, 1, '-')
+
     def test_unctrl(self):
         self.assertEqual(curses.unctrl(b'A'), b'A')
         self.assertEqual(curses.unctrl('A'), b'A')

Misc/NEWS.d/next/Library/2026-08-10-20-05-55.gh-issue-155499._P6SDK.rst

@@ -0,0 +1,3 @@
+:meth:`curses.window.border` and :meth:`curses.window.box` no longer raise
+:exc:`TypeError` when ``0`` is mixed with a one-character string or bytes
+argument, restoring the documented default-character sentinel behavior.

Modules/_cursesmodule.c

@@ -2705,7 +2705,7 @@ _curses_window_border_impl(PyCursesWindowObject *self, PyObject *ls,
     }
     if (use_wide) {
         for (i = 0; i < 8; i++) {
-            if (objs[i] == NULL) {
+            if (objs[i] == NULL || (types[i] == 1 && ch[i] == 0)) {
                 wch_p[i] = NULL;  /* use the default character */
             }
             else if (types[i] == 2) {
@@ -2775,13 +2775,17 @@ _curses_window_box_impl(PyCursesWindowObject *self, int group_right_1,
         }
     }
     if (t1 == 2 || t2 == 2) {
-        if (t1 != 2 || t2 != 2) {
+        int t1_ok = (t1 == 2) || (t1 == 1 && ch1 == 0);
+        int t2_ok = (t2 == 2) || (t2 == 1 && ch2 == 0);
+        if (!t1_ok || !t2_ok) {
             PyErr_SetString(PyExc_TypeError,
                             "box() cannot mix integer or bytes characters "
                             "with wide string characters");
             return NULL;
         }
-        int rtn = wborder_set(self->win, &wch1, &wch1, &wch2, &wch2,
+        const cchar_t *wch1_p = (t1 == 2) ? &wch1 : NULL;
+        const cchar_t *wch2_p = (t2 == 2) ? &wch2 : NULL;
+        int rtn = wborder_set(self->win, wch1_p, wch1_p, wch2_p, wch2_p,
                               NULL, NULL, NULL, NULL);
         return curses_window_check_err(self, rtn, "wborder_set", "box");
     }