Error on conditional initialization of Final class variables

[nelsyeung]

Bug Report

Conditional initialization of a Final class variable is reported as a reassignment error on elif/else branches.

To Reproduce

tmp.py:

from typing import Final, Optional


class Foo:
    a: Final[Optional[int]]

    def __init__(self, v: int) -> None:
        if v == 0:
            self.a = None
        else:
            self.a = v  # error: Cannot assign to final attribute "a"  [misc]

Expected Behavior

No errors on the first correct assignment of a final variable regardless of branching.

Actual Behavior

$ mypy tmp.py
tmp.py:11: error: Cannot assign to final attribute "a"  [misc]
Found 1 error in 1 file (checked 1 source file)

Reported error: Cannot assign to final attribute "a" [misc] during the else clause, even though it's only the first assignment of the variable.

Your Environment

• Mypy version used: 0.910

• Mypy command-line flags: mypy tmp.py

• Mypy configuration options from pyproject.toml (and other config files):

  [tool.mypy]
  show_error_codes = true

• Python version used: 3.9.5

• Operating system and version: macOS 11.4

Related

• Pyright has already fixed this issue: Error on conditional initialization of Final class variables microsoft/pyright#1917

[randolf-scholz]

Match-case statements are also affected: https://mypy-play.net/?mypy=master&python=3.11&gist=f0b81218a489585562acd1d8468c2f26

from typing import Final

class Foo:
    bar: Final[tuple[int, ...]]
    
    def __init__(self, bar: None | int | tuple[int, ...]) -> None:
        match bar:
            case None:
                self.bar = (-1,)
            case int():
                self.bar = (bar,)  # ✘ Cannot assign to final attribute "bar"  [misc]
            case tuple():
                self.bar = bar     # ✘ Cannot assign to final attribute "bar"  [misc]

[merc1031]

I have a similar issue / conundrum.
Is there anyway to (forward) declare the type of a variable that will be used in and exhaustive match?

https://mypy-play.net/?mypy=master&python=3.11&gist=10f92cdd6e414525c63d25ae251f9afa

def __init__(self, foo: None | int | tuple[int, ...]) -> tuple[int, ...]:
    bar: Final[tuple[int, ...]]  # ✘ Final name must be initialized with a value  [misc]
    match foo:
        case None:
            bar = (-1,)  # ✘ Cannot assign to final attribute "bar"  [misc]
        case int():
            bar = (foo,)  # ✘ Cannot assign to final attribute "bar"  [misc]
        case tuple():
            bar = foo  # ✘ Cannot assign to final attribute "bar"  [misc]
        case tuple():
            bar = foo  # ✘ Cannot assign to final attribute "bar"  [misc]

    return bar

[randolf-scholz]

@merc1031 Try

from typing import Final

class Foo:
    bar: Final[tuple[int, ...]]

    def __init__(self, foo: None | int | tuple[int, ...]) -> None:
        bar: tuple[int, ...]
        match foo:
            case None:
                bar = (-1,)
            case int():
                bar = (foo,)
            case tuple():
                bar = foo
            case tuple():
                bar = foo
    
        self.bar = bar

[merc1031]

@randolf-scholz
If you notice my use case is without the class / instance variable. Only a function.

In a larger context it would be to protect from someone modifying bar before the end of the function

https://mypy-play.net/?mypy=master&python=3.11&gist=372aa35d3e4f02d35aabe38a3b709473

from typing import Final


def __init__(self, foo: None | int | tuple[int, ...]) -> tuple[tuple[int, ...], bool]:
    bar: Final[tuple[int, ...]]
    match foo:
        case None:
            bar = (-1,)
        case int():
            bar = (foo,)
        case tuple():
            bar = foo
        case tuple():
            bar = foo
    # more code happens here
    # protect from bar being reassigned
    return bar, True

[Avasam]

I feel like there's 2 issues here.

1. is the assignement to a Final attribute in a class' __init__. Duplicate of Allow Final attributes declared in class body if initialized in ctor #8982
2. is the conditional assignement to a Final variable when all code paths include said assignement. And a ternary is not possible.
   For example:

if pytorch_is_available:
    import torch

    pytorch_gpu_is_available: Final = torch.cuda.is_available()
else:
    # We only assign once
    pytorch_gpu_is_available: Final = False  # type: ignore[misc]