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

Brett Cannon brett at python.org
Mon Nov 13 16:46:22 EST 2017

On Mon, Nov 13, 2017, 13:37 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.

The internal and stdlib AST are generated from the AST definition which is written in a DEAL. The conversion code is also auto-generated. (The devguide has the details.)

I can't see much complexity being added if we were to decide to standardise the Python representation.

Do you have a specific example in a recent release where a change was made that you disapproved of? Those of us who have mutated the AST have tried to not be gratuitous in the changes to begin with while also not letting the AST make maintaining Python harder. Plus there are external libraries like typed_ast and asteroid that try to make the AST uniform across releases so we don't have to worry quite as much about this explicitly.

-brett

-- Greg

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