gh-129858: Special syntax error for elif block after else (#129902)

Author: swfarnsworth

Doc/whatsnew/3.14.rst

@@ -348,6 +348,21 @@ Improved error messages
          ^^^^^^^
      ValueError: too many values to unpack (expected 3, got 4)
 
+* :keyword:`elif` statements that follow an :keyword:`else` block now have a specific error message.
+  (Contributed by Steele Farnsworth in :gh:`129902`.)
+
+  .. code-block:: pycon
+
+      >>> if who == "me":
+      ...     print("It's me!")
+      ... else:
+      ...     print("It's not me!")
+      ... elif who is None:
+      ...     print("Who is it?")
+      File "<stdin>", line 5
+        elif who is None:
+        ^^^^
+      SyntaxError: 'elif' block follows an 'else' block
 
 * If a statement (:keyword:`pass`, :keyword:`del`, :keyword:`return`,
   :keyword:`yield`, :keyword:`raise`, :keyword:`break`, :keyword:`continue`,

Grammar/python.gram

@@ -1422,6 +1422,7 @@ invalid_elif_stmt:
 invalid_else_stmt:
     | a='else' ':' NEWLINE !INDENT {
         RAISE_INDENTATION_ERROR("expected an indented block after 'else' statement on line %d", a->lineno) }
+    | 'else' ':' block 'elif' { RAISE_SYNTAX_ERROR("'elif' block follows an 'else' block")}
 invalid_while_stmt:
     | 'while' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a='while' named_expression ':' NEWLINE !INDENT {

Lib/test/test_syntax.py

@@ -948,6 +948,18 @@
      ...
    SyntaxError: 'break' outside loop
 
+elif can't come after an else.
+
+    >>> if a % 2 == 0:
+    ...     pass
+    ... else:
+    ...     pass
+    ... elif a % 2 == 1:
+    ...     pass
+    Traceback (most recent call last):
+      ...
+    SyntaxError: 'elif' block follows an 'else' block
+
 Misuse of the nonlocal and global statement can lead to a few unique syntax errors.
 
    >>> def f():

Misc/NEWS.d/next/Core_and_Builtins/2025-02-12-01-36-13.gh-issue-129858.M-f7Gb.rst

@@ -0,0 +1 @@
+``elif`` statements that follow an ``else`` block now have a specific error message.