gh-153171: Produce specialized syntax errors for 'not' after an operator in more positions (GH-153175)

Move the invalid_factor rule from term to factor and the
invalid_arithmetic rule from shift_expr to sum, so that they are
tried at all positions where the invalid construct can occur.
Previously a generic "invalid syntax" error was reported for
constructs like "1 << 2 + not x" or "1 * + not x".

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: serhiy-storchaka

Grammar/python.gram

@@ -835,7 +835,6 @@ bitwise_and[expr_ty]:
 shift_expr[expr_ty]:
     | a=shift_expr '<<' b=sum { _PyAST_BinOp(a, LShift, b, EXTRA) }
     | a=shift_expr '>>' b=sum { _PyAST_BinOp(a, RShift, b, EXTRA) }
-    | invalid_arithmetic
     | sum
 
 # Arithmetic operators
@@ -844,6 +843,7 @@ shift_expr[expr_ty]:
 sum[expr_ty]:
     | a=sum '+' b=term { _PyAST_BinOp(a, Add, b, EXTRA) }
     | a=sum '-' b=term { _PyAST_BinOp(a, Sub, b, EXTRA) }
+    | invalid_arithmetic
     | term
 
 term[expr_ty]:
@@ -852,14 +852,14 @@ term[expr_ty]:
     | a=term '//' b=factor { _PyAST_BinOp(a, FloorDiv, b, EXTRA) }
     | a=term '%' b=factor { _PyAST_BinOp(a, Mod, b, EXTRA) }
     | a=term '@' b=factor { CHECK_VERSION(expr_ty, 5, "The '@' operator is", _PyAST_BinOp(a, MatMult, b, EXTRA)) }
-    | invalid_factor
     | factor
 
 factor[expr_ty] (memo):
     | '+' a=factor { _PyAST_UnaryOp(UAdd, a, EXTRA) }
     | '-' a=factor { _PyAST_UnaryOp(USub, a, EXTRA) }
     | '~' a=factor { _PyAST_UnaryOp(Invert, a, EXTRA) }
     | power
+    | invalid_factor
 
 power[expr_ty]:
     | a=await_primary '**' b=factor { _PyAST_BinOp(a, Pow, b, EXTRA) }

Lib/test/test_syntax.py

@@ -2268,6 +2268,22 @@
 Traceback (most recent call last):
 SyntaxError: 'not' after an operator must be parenthesized
 
+>>> 1 << 2 + not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> 1 >> 2 * not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> 3 * + not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> 3 ** - not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
 # Check that we don't introduce misleading errors
 >>> not 1 */ 2
 Traceback (most recent call last):

Misc/NEWS.d/next/Core_and_Builtins/2026-07-06-18-00-00.gh-issue-153171.errloc1.rst

@@ -0,0 +1,3 @@
+Improve syntax error messages for misplaced ``not`` after an operator
+in contexts where a generic "invalid syntax" error was previously reported,
+such as ``1 << 2 + not x`` and ``1 * + not x``.