Specialized syntax error messages are not produced in all positions where the invalid construct can occur

Feature or enhancement

Several rules for generating specialized syntax error messages are attached to a grammar rule in which the invalid construct can only occur at some of its possible positions. When the same construct occurs at a deeper position, the parser falls back to a generic SyntaxError: invalid syntax.

invalid_factor catches a unary operator followed by not. It is an alternative of term, so it is only tried where a term starts. But its construct occurs wherever a factor is expected -- also as the right operand of *, /, %, //, @ and **:

>>> + not x
SyntaxError: 'not' after an operator must be parenthesized
>>> 1 * + not x
SyntaxError: invalid syntax
>>> 2 ** + not x
SyntaxError: invalid syntax

invalid_arithmetic catches a binary arithmetic operator followed by not. It is an alternative of shift_expr, so it misses the construct in the right operand of << and >> (a sum position):

>>> 1 + not x
SyntaxError: 'not' after an operator must be parenthesized
>>> 1 << 2 + not x
SyntaxError: invalid syntax

The fix is to attach each error rule to the rule matched at all positions where its construct can occur:

• move invalid_factor from term to factor;
• move invalid_arithmetic from shift_expr to sum.

After this change all the examples above produce the specialized error messages, and all previously produced messages are unchanged.

The placement of invalid_factor and invalid_arithmetic dates from their introduction in bpo-24612 (GH-28170), which attached both symmetrically to the rule containing the binary operators, one level above the operand rule where the construct can actually occur.

There are a few related gaps which require modifying the error rules themselves rather than moving them (1 << not x, lambda x=: 0, def f[T=]: pass, x = 1 + *y); they are left for separate issues.

Linked PRs

• gh-153175
• gh-153193

Beginnen Sie mit den im Issue genannten Grammatikregeln: invalid_factor, invalid_arithmetic, term, factor, shift_expr und sum. Verschieben Sie jede Fehlerregel in die angegebene Operandenregel und prüfen Sie anschließend die aufgeführten interaktiven Beispiele, um zu bestätigen, dass spezialisierte Meldungen erscheinen und bestehende Meldungen unverändert bleiben.