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

Greg Ewing greg.ewing at canterbury.ac.nz
Mon Nov 13 16:36:32 EST 2017

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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