Drop backend=... argument.

[tomchristie]

Now that we have async autodetection, I think we ought to consider dropping the backend argument from AsyncClient.__init__(...), since it's not really required.

If anything it'd be better to find out if/when there are any cases that sniffio isn't able to correctly detect, and fix them, rather than having users be able to flick an override switch.

(It'd also potentially make it easier to have a clear split if we wanted to factor the dispatch out into an entirely independant package at some point, since we'd want the backend implementations to reside in the core networking package, rather than in httpx.)

[dalf]

Note: I use a custom Backend to record the certificates of the HTTPS requests.
This backend behaves as a proxy to a real backend, but record the ssl.SSLObject on the way.

Code:

• https://github.com/dalf/searx-stats2/blob/master/searxstats/common/ssl_info.py#L85
• https://github.com/dalf/searx-stats2/blob/master/searxstats/common/http.py#L70

[EDIT] As I understand, the code will be something like this (using the private API):

from httpx._backend.auto import AutoBackend
from httpx._dispatch.connection_pool import ConnectionPool

class CustomBackend(AutoBackend):
    async def open_tcp_stream(
        self,
        hostname: str,
        port: int,
        ssl_context: typing.Optional[ssl.SSLContext],
        timeout: Timeout,
    ) -> BaseSocketStream:
        value = await super().open_tcp_stream(hostname, port, ssl_context, timeout)
        # use value.stream_reader._transport.get_extra_info('ssl_object')
        return value

async def f(*args, **kwargs):
    backend = CustomBackend()
    dispatch = ConnectionPool(backend=backend, http2=True)
    async with AsyncClient(dispatch=dispatch) as client:
       ....

[tomchristie]

@dalf - That's a really useful (surprising) bit of feedback, thanks!

(And yes, you've got the right approach there, but make sure you're pinning your dependencies tightly if you're relying on private API!)