gh-46927: Prevent readline from overriding environment

Lib/test/test_readline.py

@@ -431,6 +431,16 @@ def my_hook():
         readline.set_pre_input_hook(my_hook)
         self.assertIs(readline.get_pre_input_hook(), my_hook)
 
+    def test_environment_is_not_modified(self):
+        # os.environ contains enviroment at the time "os" module was loaded, so
+        # before the "readline" module is loaded.
+        original_env = dict(os.environ)
+
+        # Force refresh of os.environ and make sure it is the same as before the
+        # refresh.
+        os.reload_environ()
+        self.assertEqual(dict(os.environ), original_env)
+
 
 @unittest.skipUnless(support.Py_GIL_DISABLED, 'these tests can only possibly fail with GIL disabled')
 class FreeThreadingTest(unittest.TestCase):

Misc/NEWS.d/next/Core_and_Builtins/2025-05-07-18-31-31.gh-issue-46927.sF02gj.rst

@@ -0,0 +1,2 @@
+Prevent :mod:`readline` from overriding the ``COLUMNS`` and ``LINES``
+environment variables, as values are not updated on terminal resize.

Modules/readline.c

@@ -1349,6 +1349,13 @@ setup_readline(readlinestate *mod_state)
     /* The name must be defined before initialization */
     rl_readline_name = "python";
 
+#if !defined(__APPLE__)
+    /* Prevent readline from changing environment variables such as LINES and
+     * COLUMNS.
+     */
+    rl_change_environment = 0;
+#endif
+
     /* the libedit readline emulation resets key bindings etc
      * when calling rl_initialize.  So call it upfront
      */