PEP-563 interacts in weird ways with constant folding. running the following code:

from __future__ import annotations

def f(a: 5 + 7) -> a ** 39:
    return 12

print(f.__annotations__)

I would expect this output:

{'a': '5 + 7', 'return': 'a ** 39'}

But I get:

{'a': '12', 'return': 'a ** 39'}

Łukasz, should we disable AST optimizations in annotations if "from __future__ import annotations" is used?

Yo be honest, this would add a lot of complexity as "from __future__ import annotations" is a compiler directive (CO_FUTURE_ANNOTATIONS) and bringing this to the AST level would be somehow disrupting.

I would be -1 to disable AST optimizations in annotations.

I don't have a particularly deep opinion on what should be done, just a bit of weirdness I hit upon while implementing the PEP in PyPy. fwiw, we implement it as an AST transformer that the compiler runs before running the optimizer to make sure that the AST optimizations don't get applied to annotions. The transformer replaces all annotations with a Constant ast node, containing the unparsed string.

> I don't have a particularly deep opinion on what should be done, just a bit of weirdness I hit upon while implementing the PEP in PyPy. fwiw, we implement it as an AST transformer that the compiler runs before running the optimizer to make sure that the AST optimizations don't get applied to annotions. The transformer replaces all annotations with a Constant ast node, containing the unparsed string.

I have given it a try in PR 17866 and it was not as invasive as I imagine, so if Łukasz agrees, we can go ahead :)

New changeset d112c60 by Pablo Galindo in branch 'master':
bpo-39220: Do not optimise annotation if 'from __future__ import annotations' is used (GH-17866)
d112c60