Consider making 'sniffio' dependency optional

[rc-mattschwager]

Hi there!

Thank you for the excellent httpx library. I use it everyday, and it makes async programming in Python a joy.

In the interest of dependency minimization, I would like to purpose making the sniffio dependency/functionality optional. Correct me if I'm wrong here, but I suspect 99% of users are using the stdlib async implementation, so sniffio is unnecessary. Further, users leveraging something like trio or curio are likely power users who know what they're doing, and can jump through the additional hoop of optional dependencies.

httpx appears to have a history of dependency minimization (#2165, #2252), and making judicious use of optional dependencies (https://github.com/encode/httpx#dependencies). Further, sniffio usage is minimal, and thus shouldn't be too difficult to manage as an optional dependency:

• https://github.com/encode/httpx/blob/9baf3a6cd2/httpx/_transports/asgi.py#L18
• https://github.com/encode/httpx/blob/9baf3a6cd2/httpx/_utils.py#L387
• https://github.com/encode/httpx/blob/9baf3a6cd2/tests/concurrency.py#L12

Reviewing the original PR (#585) it doesn't appear that there was any major driving factor in its inclusion, at least that I can see.

Let me know if I'm missing anything, and I'm happy to open the discussion on this subject.

[lovelydinosaur]

The sniffio package is a super tiny dependency, so the thing to do here would be to figure out how to replicate the auto-detection of "are we in an asyncio or a trio context", without using the package.

From a bit of digging I think we can do this ourselves, while still relying on public API...

def determine_async_context():
    try:
        if asyncio.current_task() is not None:
            return 'asyncio'
    except RuntimeError:
        pass

    try:
        import trio
        if trio.lowlevel.current_task() is not None:
            return 'trio'
    except (RuntimeError, ImportError):
        pass

    raise RuntimeError("determine_async_context called, but no task is running")

I think that switching around to something like that could probably allow us to remove our sniffio dependancy.

As it happens, we've got a much more substantial async dependancy in the form of anyio, which httpcore uses for it's async implementation. However let's discuss that separately.

[rc-mattschwager]

Ahh, that's an interesting approach. I was envisioning something different, but that would work as well.

I was thinking something more along the lines of:

try:
    import sniffio
except ImportError:
    sniffio = None

...

if sniffio is not None and sniffio.current_async_library() ...
    ...

Then there could be an optional dependency along the lines of httpx[sniffio].

I think either approach would work well to achieve the spirit of the original request. What do you think of the tradeoffs between these two approaches?