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[Python-Dev] Standardise the AST (Re: PEP 563: Postponed Evaluation of Annotations)

Nathaniel Smith njs at pobox.com
Mon Nov 13 18:40:22 EST 2017 
Can you give any examples of problems caused by the ast not being
standardized? The original motivation of being able to distinguish between
  foo(x: int)
  foo(x: "int")
isn't very compelling – it's not clear it's a problem in the first place,
and even if it is then all we need is some kind of boolean flag, not an ast
standard.

On Nov 13, 2017 13:38, "Greg Ewing" <greg.ewing at canterbury.ac.nz> wrote:

> Guido van Rossum wrote:
>
>> But Python's syntax changes in nearly every release.
>>
>
> The changes are almost always additions, so there's no
> reason why the AST can't remain backwards compatible.
>
> the AST level ... elides many details (such as whitespace and parentheses).
>>
>
> That's okay, because the AST is only expected to
> represent the semantics of Python code, not its
> exact lexical representation in the source. It's
> the same with Lisp -- comments and whitespace have
> been stripped out by the time you get to Lisp
> data.
>
> Lisp had almost no syntax so I presume the mapping to data structures was
>> nearly trivial compared to Python.
>>
>
> Yes, the Python AST is more complicated, but we
> already have that much complexity in the AST being
> used by the compiler.
>
> If I understand correctly, we also have a process
> for converting that internal structure to and from
> an equally complicated set of Python objects, that
> isn't needed by the compiler and exists purely for
> the convenience of Python code.
>
> I can't see much complexity being added if we were
> to decide to standardise the Python representation.
>
> --
> Greg
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