Clarify `Final` assignment

Typing
1

from Final semantics:

  • There must be exactly one assignment to a final name.

this leads to the following contradiction:
Code sample in basedpyright playground

from typing import Final

def random_coin_flip() -> bool:
    return True  # pre-computed using a real coin

a: Final = 1 if random_coin_flip() else 0  # valid, a single assignment

b: Final[int]  # the type could potentially be inferred here, but lets not say that it is mandatory to support

if random_coin_flip():
    b = 1
else:
    b = 0  # invalid, a secondary assignment

we can see that the semantics of a and b are identical, yet we observe different type checking behaviour

i donā€™t think that b should have this error

5 Likes
2

Do we even need to support module-scope Final without an initializer?

from typing import Final

A: Final
A = 1

Currently only pyright and ty allow this; mypy, zuban, pyrefly, and pycroscope reject it saying that a Final variable needs an initializer.

I know the spec explicitly allows Final instance attributes to be declared in class scope without an initializer, but that seems different and more useful.

1 Like
3

Itā€™s useful when importing backports or optional accelerators to have constants defined based on which actual backend for something is in use.

Iā€™m in favor of the change, but Iā€™m not sure the wording is as precise as it could be.

This is not the only place with typing where taking into account actual code flow would improve specified behavior. I think the proposed wording and specification change is fine for now, but thereā€™s significantly more that could be improved in the same vein which might later result in wanting more precise language.

1 Like
4

The same branchy-initialization case that the spec change aims to allow, effectively requires an initializer-less Final declaration; otherwise it would run afoul of ā€œonly one Final declaration allowedā€.

5

It would indeed be very useful, as quite often I do things like:

if random_coin_flip():
    b = 1
    # Some extra stuff
else:
    b = 0
    # Some other extra stuff

But using Final in an assignment-less declaration isnā€™t to my taste, personally.