fix: Create py.typed

[kongzii]

Hi, just a small fix: without this, mypy doesn't consider langfuse's typing.

Given the following test file

from langfuse.decorators import observe


@observe()
def test(x: int) -> str:
    return str(x)


y = test(5)
reveal_type(y)

with this change, mypy looks like this:

test.py:9: error: Argument 1 to "test" has incompatible type "str"; expected "int"  [arg-type]
test.py:10: note: Revealed type is "builtins.str"

without this change (with the latest langfuse version), it looks like this:

test.py:10: note: Revealed type is "Any"
Success: no issues found in 1 source file

---

I see there is some discussion already in https://github.com/langfuse/langfuse/issues/2169, but it got stalled.

It is true that if I just run mypy on this repository, there are many complaints:

main $ mypy . | wc -l                                                                                                                                                                                                                           [10:55:22]
     517

So it would require a big effort to fix the whole library for mypy, but on the other hand, it's a shame that currently, any project that uses @observe is losing type safety, which can be fixed by this change.

As a hot fix, without this change, having this wrapper in our code repository around the langfuse's observer seems like a way to go, but it's not a great solution:

from langfuse.decorators.langfuse_decorator import ( 
    langfuse_context,
    observe as original_observe,
)
from typing import TypeVar, ParamSpec, Optional, Literal, Callable, Iterable, Any

P = ParamSpec("P")
R = TypeVar("R")


def observe(
    name: Optional[str] = None,
    as_type: Optional[Literal["generation"]] = None,
    capture_input: bool = True,
    capture_output: bool = True,
    transform_to_string: Optional[Callable[[Iterable[Any]], str]] = None,
) -> Callable[[Callable[P, R]], Callable[P, R]]:
    casted: Callable[[Callable[P, R]], Callable[P, R]] = original_observe(
        name=name,
        as_type=as_type,
        capture_input=capture_input,
        capture_output=capture_output,
        transform_to_string=transform_to_string,
    )
    return casted

[CLAassistant]

All committers have signed the CLA.

[hassiebp]

Thanks again for raising this! Closing this as stale / no longer relevant in Python SDK v3. If you continue to see issues, please open a Github issue 👍🏾

[kongzii]

@hassiebp Why close this instead of merging this?

Of course, this is still very relevant in Python SDK v3 as well. Without this, mypy just won't consider the typing from the Langfuse library. See:

[kongzii]

Created the issue https://github.com/langfuse/langfuse/issues/7880.