Issue #25203: Failed readline.set_completer_delims() no longer left the

 module in inconsistent state.

Misc/NEWS

@@ -37,6 +37,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #25203: Failed readline.set_completer_delims() no longer left the
+  module in inconsistent state.
+
 - Issue #19143: platform module now reads Windows version from kernel32.dll to
   avoid compatibility shims.
 

Modules/readline.c

@@ -355,10 +355,11 @@ set_completer_delims(PyObject *self, PyObject *args)
     /* Keep a reference to the allocated memory in the module state in case
        some other module modifies rl_completer_word_break_characters
        (see issue #17289). */
-    free(completer_word_break_characters);
-    completer_word_break_characters = strdup(break_chars);
-    if (completer_word_break_characters) {
-        rl_completer_word_break_characters = completer_word_break_characters;
+    break_chars = strdup(break_chars);
+    if (break_chars) {
+        free(completer_word_break_characters);
+        completer_word_break_characters = break_chars;
+        rl_completer_word_break_characters = break_chars;
         Py_RETURN_NONE;
     }
     else