gh-146495: Improve SyntaxError message for && and || operators

[Aniketsy]

fixes #146495

@Aniketsy I would prefer that a parser expert takes care of that. This is likely a non-trivial change so to make it easier to review, it would be better if a core dev or someone familiar with the parser takes care of this.

As there was not any pr for this for long time, so i thought to give a try for this as @skirpichev have shared the patch towards fix. Hoping it does not create much pain to review for members. Thank you !

• Issue: Consider specialized syntax error for && and || #146495

[sobolevn]

I like the idea! Before it was:

[sobolevn]

Can you please also assert the error location, it should be the second | or & char.

[Aniketsy]

thanks for the review, i've updated with error location, please let me know if we need any further improvements

[pablogsal]

Thanks for the PR @Aniketsy! Unfortunately this is not the correct approach for such an error message. See my comment below

[pablogsal]

This is not a good approach for this: _Pypegen_set_syntax_error is a generic functions to set a syntax erroor it should absolutely not know about generic error conditions that are derived from thr Grammar semantics, only the tokenizer middle language (that's why out checks for INDENT/DEDENT). The correct way to raise custom syntax errors is custom grammar rules that trigger on the second pass via invalid_ rules

[skirpichev]

@Aniketsy, I think you can address this as:

diff --git a/Grammar/python.gram b/Grammar/python.gram
index 9bf3a67939f..406201e12ae 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -827,6 +827,7 @@ bitwise_xor[expr_ty]:
 
 bitwise_and[expr_ty]:
     | a=bitwise_and '&' b=shift_expr { _PyAST_BinOp(a, BitAnd, b, EXTRA) }
+    | invalid_bitwise_and
     | shift_expr
 
 shift_expr[expr_ty]:
@@ -1638,3 +1639,6 @@ invalid_type_params:
         RAISE_SYNTAX_ERROR_STARTING_FROM(
             token,
             "Type parameter list cannot be empty")}
+
+invalid_bitwise_and:
+    | a=bitwise_and b='&' c='&' { RAISE_SYNTAX_ERROR_KNOWN_RANGE(b, c, "invalid syntax.  Maybe you meant 'and' or '&' instead of '&&'") }

(ditto for bitwise_or).

This works for me:

Python 3.16.0a0 (heads/main-dirty:e28a2f49304, Jun  5 2026, 04:45:37) [GCC 14.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a, b = 1, 2
>>> a && b
  File "<python-input-1>", line 1
    a && b
      ^^
SyntaxError: invalid syntax.  Maybe you meant 'and' or '&' instead of '&&'

See InternalDocs/changing_grammar.md on changing grammar.

[Aniketsy]

@pablogsal @skirpichev thanks for the review and the suggestion, i've updated

[bedevere-app]

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