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

Greg Ewing greg.ewing at canterbury.ac.nz
Sun Nov 12 17:39:05 EST 2017

Guido van Rossum wrote:
> The PEP answers that clearly (under Implementation):
>  > If an annotation was already a string, this string is preserved
>  > verbatim.

This bothers me, because it means the transformation from
what you write in the source and the object you get at
run time is not reversible. Something interpreting the
annotations as Python expressions at run time has no way
to know whether a Python expression was written in the
source or a string literal whose contents happen to look
like an expression.

I still think that the run-time form of a non-evaluated
annotation should be some form of AST. That's been rejected
on the grounds that the structure of an AST is considered
an implementation detail that can change between Python

However, that in itself seems like a bad thing to me.
There *should* be a standard and stable form of AST
provided that doesn't change unless the syntax of Python
changes. It doesn't have to be the same as what's used
internally by the compiler.

Proponents of Lisp point to the advantages of easily
being able to express Lisp programs using Lisp data
structures. There would also be benefits in having a
standard way to represent Python programs using Python
data structures.


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