bpo-45759: Better error messages for non-matching 'elif'/'else' statements

Grammar/python.gram

@@ -126,6 +126,7 @@ 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
@@ -1174,6 +1175,10 @@ 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])+ &&':'
     | [ASYNC] 'with' '(' ','.(expressions ['as' star_target])+ ','? ')' &&':'

Lib/test/test_syntax.py

@@ -1280,6 +1280,28 @@
     ...     ...
     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
 """
 
 import re
@@ -1295,8 +1317,8 @@ def _check_error(self, code, errtext,
                      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:
@@ -1320,6 +1342,22 @@ def _check_error(self, code, errtext,
         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 = ' ')",
@@ -1582,6 +1620,25 @@ def test_syntax_error_on_deeply_nested_blocks(self):
 """
         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",
+                "invalid syntax")
+
 
 def load_tests(loader, tests, pattern):
     tests.addTest(doctest.DocTestSuite())

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.