Support for short-circuit evaluation

[mmzeynalli]

Bug Report

Mypy fails to evaluate variable type in short-circuit evaluation.

To Reproduce

class PortfolioQueryBuilder:
    def __init__(self, user_username: Optional[str] = None) :
        self.user_username = user_username

    def build_query(self):
        '''rest of the code/handling here'''

class Token:
    def __init__(self, username: str):
        self.username = username

auth = Token('username')
# auth = None
builder = PortfolioQueryBuilder(user_username=auth and auth.username)

Expected Behavior

It should not give an error, as line auth and auth.username always returns either None or username string, because of short-circuit evaluation.

Actual Behavior

When I run mypy, I get this error:

error: Argument "user_username" to "PortfolioQueryBuilder" has incompatible type "Token | str | None"; expected "str | None"  [arg-type]

Your Environment

• Mypy version used: mypy 1.9.0 (compiled: yes)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
plugins = ["sqlalchemy.ext.mypy.plugin", "pydantic.mypy"]
exclude = [
    "^admin\\.py$"
]

• Python version used: 3.11.5

[JelleZijlstra]

Mypy is technically correct here; it could be a subclass of Token that overrides __bool__ to return False.

A bit arcane but I think this sort of pattern is better avoided in type-safe code.

[erictraut]

Consistent with Jelle's insight, the behavior changes if you mark the Token class as @final.

[JelleZijlstra]

Indeed, thanks @erictraut: https://mypy-play.net/?mypy=latest&python=3.10&gist=c808a9a03bd113724835d66cccdd062e. With that I don't think there's anything left to do here; mypy's behavior is correct and @final can be used to work around this behavior if it's a problem.

[mmzeynalli]

I see, @final did indeed work. Thanks

[mmzeynalli]

Okay, I have a little different circumstance, which gives the same error:

from typing import final

from fastapi import Depends
from pydantic import BaseModel


@final
class Token(BaseModel):
    username: str
    exp: Optional[int] = None
    user_type: int


OptionalAuthUser = Annotated[Token | None, Depends(get_optional_user_access)]


@router.get(
    '',
    summary='Get portfolio list',
    status_code=status.HTTP_200_OK,
)
async def get_all_profiles_portfolios(
    db: DbDependency,
    auth: OptionalAuthUser,
):
    builder = PortfolioQueryBuilder(user_username=auth and auth.username)
    '''rest of the code'''

How should I act here, considering Token if wrapped into Annotated?

[JelleZijlstra]

You use TokenPayload but don't show that definition in your example. I would recommend rewriting as auth.username if auth is not None else None.

[mmzeynalli]

My bad, fixed it. I did that before, but then I changed the code to as shown, however mypy complains. It does not hinder my work at all, but I would still like to know how to handle this, if I were to use short-circuit.

[KotlinIsland]

@mmzeynalli I don't see any error, maybe you could provide a more complete example?

from typing import final, Optional, Annotated

from fastapi import Depends
from pydantic import BaseModel


@final
class Token(BaseModel):
    username: str
    exp: Optional[int] = None
    user_type: int

def get_optional_user_access(): ...

OptionalAuthUser = Annotated[Token | None, Depends(get_optional_user_access)]


async def get_all_profiles_portfolios(
        auth: OptionalAuthUser,
):
    builder: str | None = auth and auth.username
    '''rest of the code'''

> mypy test.py
Success: no issues found in 1 source file