Passing a custom transport disables environment proxies

[RobertCraigie]

This behaviour was added in #1122 and I'm curious why transport was included?

In anthropic-python we recently released an update that includes sending some TCP keep alive packets but incidentally this also broke environment proxies.

Additionally, it looks like the code used to read these environment variables is all private? How would we support both environment variables & a custom transport? Would we have to essentially duplicate the logic in _get_proxy_map() and pass it to mounts?

[jks15satoshi]

I was also confused with this behaviour. As for now, I have to manually pass environment proxies to mounts to make sure the environment proxies work with my custom transport:

from httpx import AsyncClient
from httpx._utils import get_environment_proxies

client = AsyncClient(
	mounts={
		pattern: CustomTransport(proxy=proxy, ...)
		for pattern, proxy in get_environment_proxies().items()
	},
	...
)

I think they should at lease describe this behaviour in the documentation if it's as designed, and what the best practice is if we do want to respect environment proxies with custom transports.