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Improve typing #492

@sobolevn

Description

@sobolevn

Hi! I've noticed several places

Transaction.__call__

Right now it is defined as:

    def __call__(self, func: typing.Callable) -> typing.Callable:
        """
        Called if using `@database.transaction()` as a decorator.
        """

        @functools.wraps(func)
        async def wrapper(*args: typing.Any, **kwargs: typing.Any) -> typing.Any:
            async with self:
                return await func(*args, **kwargs)

        return wrapper

databases/databases/core.py

Lines 356 to 366 in fbea46d

def __call__(self, func: typing.Callable) -> typing.Callable:
"""
Called if using `@database.transaction()` as a decorator.
"""
@functools.wraps(func)
async def wrapper(*args: typing.Any, **kwargs: typing.Any) -> typing.Any:
async with self:
return await func(*args, **kwargs)
return wrapper

Which make all wrapped function to have (*Any, **Any) -> Any type. This is not ideal.
I propose to use TypeVar('_CallableType', bound=Callable) to save the exact type.

After:

from databases import Database

db = Database('url')

async def test(a: int) -> int:
    ...

reveal_type(db.transaction()(test))
# note: Revealed type is "def (a: builtins.int) -> typing.Coroutine[Any, Any, builtins.int]"

Notice, that signature is preserved.

Transaction.__await__

Right now there's a small problem with it:

from databases import Database

db = Database('url')

async def test() -> None:
    reveal_type(await db.transaction())
    # note: Revealed type is "Any"
    # But, should be the same as the next line:
    reveal_type(await db.transaction().start())
    # note: Revealed type is "databases.core.Transaction"

I propose to use the same type for await db.transaction()

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