gh-155976: Refresh COLORS and COLOR_PAIRS in curses set_term

Lib/test/test_curses.py

@@ -3047,6 +3047,42 @@ def test_set_term(self):
         self.assertIs(curses.set_term(a), b)  # returns the previous one
         self.assertIs(curses.set_term(b), a)
 
+    def test_set_term_refreshes_lines_cols_colors(self):
+        s1 = self.make_pty()
+        s2 = self.make_pty()
+        a = curses.newterm('xterm', s1, s1)
+        if hasattr(curses, 'start_color'):
+            try:
+                curses.start_color()
+            except curses.error:
+                pass
+        b = curses.newterm('xterm-256color', s2, s2)
+        if hasattr(curses, 'start_color'):
+            try:
+                curses.start_color()
+            except curses.error:
+                pass
+
+        curses.set_term(a)
+        lines_a, cols_a = curses.LINES, curses.COLS
+        colors_a = getattr(curses, 'COLORS', None)
+
+        curses.set_term(b)
+        lines_b, cols_b = curses.LINES, curses.COLS
+        colors_b = getattr(curses, 'COLORS', None)
+
+        self.assertEqual(curses.set_term(a), b)
+        self.assertEqual(curses.LINES, lines_a)
+        self.assertEqual(curses.COLS, cols_a)
+        if colors_a is not None:
+            self.assertEqual(curses.COLORS, colors_a)
+
+        self.assertEqual(curses.set_term(b), a)
+        self.assertEqual(curses.LINES, lines_b)
+        self.assertEqual(curses.COLS, cols_b)
+        if colors_b is not None:
+            self.assertEqual(curses.COLORS, colors_b)
+
     def test_window_keeps_screen_alive(self):
         # The standard window keeps its screen alive; dropping every other
         # reference and collecting must not invalidate the window.

Misc/NEWS.d/next/C_API/2026-08-18-12-30-00.gh-issue-155976.xyz123.rst

@@ -0,0 +1 @@
+Fix an issue where :func:`curses.set_term` did not update :const:`curses.COLORS` and :const:`curses.COLOR_PAIRS` when switching terminal screens.

Modules/_cursesmodule.c

@@ -7557,6 +7557,32 @@ update_lines_cols(PyObject *private_module)
         goto error;
     }
     Py_DECREF(o);
+
+    if (curses_start_color_called) {
+        o = PyLong_FromUnsignedLongLong((unsigned long long)COLORS);
+        if (o == NULL) {
+            goto error;
+        }
+        if (PyDict_SetItemString(exposed_module_dict, "COLORS", o) < 0) {
+            goto error;
+        }
+        if (PyDict_SetItemString(private_module_dict, "COLORS", o) < 0) {
+            goto error;
+        }
+        Py_DECREF(o);
+
+        o = PyLong_FromUnsignedLongLong((unsigned long long)COLOR_PAIRS);
+        if (o == NULL) {
+            goto error;
+        }
+        if (PyDict_SetItemString(exposed_module_dict, "COLOR_PAIRS", o) < 0) {
+            goto error;
+        }
+        if (PyDict_SetItemString(private_module_dict, "COLOR_PAIRS", o) < 0) {
+            goto error;
+        }
+        Py_DECREF(o);
+    }
     Py_DECREF(exposed_module);
     return 1;