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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