bpo-45759: Better error messages for non-matching 'elif'/'else' statements (#29513)

Grammar/python.gram

@@ -124,6 +124,7 @@ simple_stmt[stmt_ty] (memo):
     | &'nonlocal' nonlocal_stmt
 
 compound_stmt[stmt_ty]:
+    | invalid_compound_stmt
     | &('def' | '@' | 'async') function_def
     | &'if' if_stmt
     | &('class' | '@') class_def
@@ -1298,6 +1299,10 @@ invalid_import_from_targets:
     | import_from_as_names ',' NEWLINE {
         RAISE_SYNTAX_ERROR("trailing comma not allowed without surrounding parentheses") }
 
+invalid_compound_stmt:
+    | a='elif' named_expression ':' { RAISE_SYNTAX_ERROR_STARTING_FROM(a, "'elif' must match an if-statement here") }
+    | a='else' ':' { RAISE_SYNTAX_ERROR_STARTING_FROM(a, "'else' must match a valid statement here") }
+
 invalid_with_stmt:
     | ['async'] 'with' ','.(expression ['as' star_target])+ NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | ['async'] 'with' '(' ','.(expressions ['as' star_target])+ ','? ')' NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }

Lib/test/test_syntax.py

@@ -1752,6 +1752,28 @@ Corner-cases that used to crash:
     Traceback (most recent call last):
     SyntaxError: positional patterns follow keyword patterns
 
+Non-matching 'elif'/'else' statements:
+
+    >>> if a == b:
+    ...     ...
+    ...     elif a == c:
+    Traceback (most recent call last):
+    SyntaxError: 'elif' must match an if-statement here
+
+    >>> if x == y:
+    ...     ...
+    ...     else:
+    Traceback (most recent call last):
+    SyntaxError: 'else' must match a valid statement here
+
+    >>> elif m == n:
+    Traceback (most recent call last):
+    SyntaxError: 'elif' must match an if-statement here
+
+    >>> else:
+    Traceback (most recent call last):
+    SyntaxError: 'else' must match a valid statement here
+
 Uses of the star operator which should fail:
 
 A[:*b]
@@ -2006,8 +2028,8 @@ class SyntaxTestCase(unittest.TestCase):
                      lineno=None, offset=None, end_lineno=None, end_offset=None):
         """Check that compiling code raises SyntaxError with errtext.
 
-        errtest is a regular expression that must be present in the
-        test of the exception raised.  If subclass is specified it
+        errtext is a regular expression that must be present in the
+        test of the exception raised. If subclass is specified, it
         is the expected subclass of SyntaxError (e.g. IndentationError).
         """
         try:
@@ -2031,6 +2053,22 @@ class SyntaxTestCase(unittest.TestCase):
         else:
             self.fail("compile() did not raise SyntaxError")
 
+    def _check_noerror(self, code,
+                       errtext="compile() raised unexpected SyntaxError",
+                       filename="<testcase>", mode="exec", subclass=None):
+        """Check that compiling code does not raise a SyntaxError.
+
+        errtext is the message passed to self.fail if there is
+        a SyntaxError. If the subclass parameter is specified,
+        it is the subclass of SyntaxError (e.g. IndentationError)
+        that the raised error is checked against.
+        """
+        try:
+            compile(code, filename, mode)
+        except SyntaxError as err:
+            if (not subclass) or isinstance(err, subclass):
+                self.fail(errtext)
+
     def test_expression_with_assignment(self):
         self._check_error(
             "print(end1 + end2 = ' ')",
@@ -2372,6 +2410,25 @@ while 1:
 """
         self._check_error(source, "too many statically nested blocks")
 
+    def test_syntax_error_non_matching_elif_else_statements(self):
+        # Check bpo-45759: 'elif' statements that doesn't match an
+        # if-statement or 'else' statements that doesn't match any
+        # valid else-able statement (e.g. 'while')
+        self._check_error(
+            "elif m == n:\n    ...",
+            "'elif' must match an if-statement here")
+        self._check_error(
+            "else:\n    ...",
+            "'else' must match a valid statement here")
+        self._check_noerror("if a == b:\n    ...\nelif a == c:\n    ...")
+        self._check_noerror("if x == y:\n    ...\nelse:\n    ...")
+        self._check_error(
+            "else = 123",
+            "invalid syntax")
+        self._check_error(
+            "elif 55 = 123",
+            "cannot assign to literal here")
+
     @support.cpython_only
     def test_error_on_parser_stack_overflow(self):
         source = "-" * 100000 + "4"

Misc/NEWS.d/next/Core and Builtins/2021-11-10-10-40-05.bpo-45759.WJoB3D.rst

@@ -0,0 +1 @@
+Improved error messages for ``elif``/``else`` statements not matching any valid statements. Patch by Jeremiah Vivian.