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Sun Jan 25 04:29:03 CET 2015 · http://mail.python.org/pipermail/python-dev/2015-January/137879.html

Have you tried it yet?

I think you have to inline dictpopulator, because dictpopulator can start
with the same tokens as test, and the parser doesn't backtrack. So it
wouldn't know how to decide whether to take the dictpopulator branch or the
set branch. If you inline it, the parser will know, because it does
something clever within the rule.

As-is, I get a lot of errors from pgen about ambiguity. This one seems to
work (but you still have to adjust the code generator of course):

dictorsetmaker: ( ((test ':' test | '**' test) (comp_for | (',' (test ':'
test | '**' test))* [','])) |
                   (test (comp_for | (',' test)* [','])) )

Also I presume you want a similar treatment for the set branch (replace
both tests with (test | '*' test).

Good luck! There's plenty of code to crib from for the code generation.

--Guido

On Sat, Jan 24, 2015 at 6:10 PM, Neil Girdhar <mistersheik at gmail.com> wrote:

> To finish PEP 448, I need to update the grammar for syntax such as
>
> {**x for x in it}
>
> and
>
> {1:2, 3:4, **a}
>
> It's been a long time since I've looked at grammars and I could really use
> the advice of an expert.  I'm considering replacing:
>
> dictorsetmaker: ( (test ':' test (comp_for | (',' test ':' test)* [','])) |
>                   (test (comp_for | (',' test)* [','])) )
>
> with:
>
> dictpopulator: test ':' test | '**' test
> dictorsetmaker: ( (dictpopulator (comp_for | (',' dictpopulator)* [','])) |
>                    (test (comp_for | (',' test)* [','])) )
>
> Am I headed in the right direction?  Of course I will need to edit
> parsermodule.c and ast.c.
>
> Best,
>
> Neil
>
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>

-- 
--Guido van Rossum (python.org/~guido)
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[Python-Dev] Any grammar experts?