Improve `SyntaxError` message for `import a as b.c`

[sobolevn]

Feature or enhancement

Right now it shows:

>>> import a as b.c
  File "<unknown>", line 1
    import a as b.c
                 ^
SyntaxError: invalid syntax

Proposed message:

>>> import a as b.c
  File "<unknown>", line 1
    import a as b.c
                ^^^
SyntaxError: cannot use import statement with attribute

Refs #123440

I have a PR ready.

Linked PRs

• gh-123539: Improve SyntaxError msg for import as with not a name #123629
• gh-123539: Add new error message changes to "Whats New" #133344

[lysnikolaou]

I'd love to review a PR for this!

[sobolevn]

It is not that easy as previous two :)
I would take some extra time to get this right!

[lysnikolaou]

Oh right. You probably have to do something more complex than what was in the other two cases. Maybe something like this would work?

dotted_as_name[alias_ty]:
    | a=dotted_name 'as' b=NAME &(',' | NEWLINE) { _PyAST_alias(a->v.Name.id, b->v.Name.id, EXTRA) }
    | invalid_dotted_as_name
    | a=dotted_name { _PyAST_alias(a->v.Name.id, NULL, EXTRA) }

invalid_dotted_as_name:
    | dotted_name 'as' a=expression {
        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use %s as import target", _PyPegen_get_expr_name(a)) }

I haven't tested it extensively, so it might break something.

[sobolevn]

Yes, this is the part I am working with. There are also cases of from a import (b as b1, c as c1), etc.

[lysnikolaou]

Right! You could do kind of the same approach in import_from_as_name for from imports I think.