Hi Stefano,
One of the problems you have is that the rule for slices has a negative
lookahead for the comma:
slices[expr_ty]:
| a=slice !',' { a }
IIRC the reason that is there is to allow "x[3,]" to be parsed.
Also, to allow "a[k=3]" you need to to create a rule that allows skipping
the "slices" part of the subscript and allow the 2nd argument of
_Py_Subscript to be NULL. For example:
| a=t_primary '[' k=kwargs ']' !t_lookahead { _Py_Subscript(a, NULL, k,
Store, EXTRA) }
Regards,
Pablo
On Fri, 16 Oct 2020 at 22:07, Stefano Borini <[email protected]>
wrote:
> Hello,
>
> I am trying to implement PEP-637, and I started modifying the parser
> and the grammar, but I don't know what I am missing.
>
> The PR is here
>
>
> https://github.com/python/cpython/compare/master...stefanoborini:PEP-637-implementation-attempt-1?expand=1
>
> It includes other stuff but the core is that I extended the Subscript
> in the asdl to accept the keyword args
>
> | Subscript(expr value, expr slice, keyword* keywords, expr_context ctx)
>
> which seems to work:
>
> >>> ast.parse('a[3]').body[0].value.keywords
> []
>
> I also added a few "productions" (I believe they are called like this,
> my compiler theory is very approximated), one of which is now
>
> primary[expr_ty]:
> ....
> | a=primary '[' b=slices c=[',' k=kwargs {k}]']' {
> _Py_Subscript(a, b, c, Load, EXTRA) }
>
> I also tried with additional formats like
>
> | a=primary '[' b=slices c=kwargs ']' { _Py_Subscript(a, b, c,
> Load, EXTRA) }
>
> or even just
>
> | a=primary '[' c=kwargs ']' { _Py_Subscript(a, NULL, c, Load, EXTRA) }
>
> but I always get a SyntaxError:
>
> >>> ast.parse('a[k=3]').body[0].value.keywords
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> File "/Users/sbo/Work/Projects/stefanoborini/cpython/Lib/ast.py",
> line 50, in parse
> return compile(source, filename, mode, flags,
> File "<unknown>", line 1
> a[k=3]
> ^
> SyntaxError: invalid syntax
>
> Note that I always recreated ast parser and pegen.c with 'make
> regen-all' and recompiled with make.
> I tried to enable debug and print the pegen.c debug, but it's a bit
> heavy and I could not immediately spot the issue. I suspect I am
> missing something somewhere.
>
> Thanks
>
>
> --
> Kind regards,
>
> Stefano Borini
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