gh-129858: Special syntax error for elif block after else

Grammar/python.gram

@@ -1396,6 +1396,7 @@ 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

@@ -918,6 +918,18 @@
      ...
    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 +1,13 @@
+`elif` statements that follow an `else` block now have a specific error message.
+```py
+>>> 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
+```