pythongh-134036: Improve error messages for invalid raise statements (python#134077)

Author: sobolevn

Grammar/python.gram

@@ -184,6 +184,7 @@ return_stmt[stmt_ty]:
     | 'return' a=[star_expressions] { _PyAST_Return(a, EXTRA) }
 
 raise_stmt[stmt_ty]:
+    | invalid_raise_stmt
     | 'raise' a=expression b=['from' z=expression { z }] { _PyAST_Raise(a, b, EXTRA) }
     | 'raise' { _PyAST_Raise(NULL, NULL, EXTRA) }
 
@@ -1287,6 +1288,11 @@ invalid_ann_assign_target[expr_ty]:
     | list
     | tuple
     | '(' a=invalid_ann_assign_target ')' { a }
+invalid_raise_stmt:
+    | a='raise' b='from' {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "did you forget an expression between 'raise' and 'from'?") }
+    | 'raise' expression a='from' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "did you forget an expression after 'from'?") }
 invalid_del_stmt:
     | 'del' a=star_expressions {
         RAISE_SYNTAX_ERROR_INVALID_TARGET(DEL_TARGETS, a) }

Lib/test/test_syntax.py

@@ -1695,6 +1695,28 @@
      ...
    SyntaxError: invalid syntax
 
+Better errors for `raise` statement:
+
+    >>> raise ValueError from
+    Traceback (most recent call last):
+    SyntaxError: did you forget an expression after 'from'?
+
+    >>> raise mod.ValueError() from
+    Traceback (most recent call last):
+    SyntaxError: did you forget an expression after 'from'?
+
+    >>> raise from exc
+    Traceback (most recent call last):
+    SyntaxError: did you forget an expression between 'raise' and 'from'?
+
+    >>> raise from None
+    Traceback (most recent call last):
+    SyntaxError: did you forget an expression between 'raise' and 'from'?
+
+    >>> raise from
+    Traceback (most recent call last):
+    SyntaxError: did you forget an expression between 'raise' and 'from'?
+
 Check that an multiple exception types with missing parentheses
 raise a custom exception only when using 'as'
 

Misc/NEWS.d/next/Core_and_Builtins/2025-05-16-09-06-38.gh-issue-134036.st2e-B.rst

@@ -0,0 +1,2 @@
+Improve :exc:`SyntaxError` message when using invalid :keyword:`raise`
+statements.