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

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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