DNS timeouts have a traceback that is confusing for users.

[tomchristie]

Prompted by this user comment... encode/httpx#2167 (comment)

When we see DNS timeouts in httpx async users will end up with a traceback that looks like this...

▶ python3 ./tooltracker.py
url count: 14
Traceback (most recent call last):
  File "/home/nem/.cache/pypoetry/virtualenvs/tool-tracker-YFqSduNZ-py3.11/lib/python3.11/site-packages/anyio/_core/_sockets.py", line 189, in connect_tcp
    addr_obj = ip_address(remote_host)
               ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/linuxbrew/.linuxbrew/opt/python@3.11/lib/python3.11/ipaddress.py", line 54, in ip_address
    raise ValueError(f'{address!r} does not appear to be an IPv4 or IPv6 address')
ValueError: 'api.github.com' does not appear to be an IPv4 or IPv6 address

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/nem/.cache/pypoetry/virtualenvs/tool-tracker-YFqSduNZ-py3.11/lib/python3.11/site-packages/httpcore/backends/asyncio.py", line 114, in connect_tcp
    stream: anyio.abc.ByteStream = await anyio.connect_tcp(
                                   ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/nem/.cache/pypoetry/virtualenvs/tool-tracker-YFqSduNZ-py3.11/lib/python3.11/site-packages/anyio/_core/_sockets.py", line 192, in connect_tcp
    gai_res = await getaddrinfo(
              ^^^^^^^^^^^^^^^^^^
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/nem/.cache/pypoetry/virtualenvs/tool-tracker-YFqSduNZ-py3.11/lib/python3.11/site-packages/httpcore/_exceptions.py", line 10, in map_exceptions
    yield
  File "/home/nem/.cache/pypoetry/virtualenvs/tool-tracker-YFqSduNZ-py3.11/lib/python3.11/site-packages/httpcore/backends/asyncio.py", line 113, in connect_tcp
    with anyio.fail_after(timeout):
  File "/home/nem/.cache/pypoetry/virtualenvs/tool-tracker-YFqSduNZ-py3.11/lib/python3.11/site-packages/anyio/_core/_tasks.py", line 119, in __exit__
    raise TimeoutError
TimeoutError

...

This looked confusing to me since it starts with the ValueError: 'api.github.com' does not appear to be an IPv4 or IPv6 address line.

Once I'd dug into the source for anyio 3.7 it made sense...

anyio/src/anyio/_core/_sockets.py

Lines 187 to 194 in f7a880f

	target_host = str(remote_host)
	try:
	addr_obj = ip_address(remote_host)
	except ValueError:
	# getaddrinfo() will raise an exception if name resolution fails
	gai_res = await getaddrinfo(
	target_host, remote_port, family=family, type=socket.SOCK_STREAM
	)

...okay, is this an IP address? If not, raise an exception and then perform DNS lookup.

So the behaviour is valid and correct. But I'm wondering if there's any scope for a UX improvement here?

Restructuring the code so that the DNS lookup doesn't occur within an except block would result in a more obvious stacktrace for this fairly common exception case.

[smurfix]

We could just do

except ValueError:
    try:
        gai_res = await getaddrinfo(
            target_host, remote_port, family=family, type=socket.SOCK_STREAM
        )
    except Exception as exc:
        raise exc from None

[agronholm]

This should be an enhancement request. Would you mind filing an issue?

[dhensen]

This issue is haunting me now. My situation is that I have a basic docker setup, I'm firing 100+ async health checks to different sites all triggering DNS lookups. Problem 1 is that my configured DNS is giving timeouts.
Problem 2 is that async httpx + anyio (as described above) throw very large and confusing tracebacks about ValueError: 'some-site.com' does not appear to be an IPv4 or IPv6 address

[dhensen]

The message in the valueError had me sidetracked a while before realizing that my real problem was DNS timeouts. Of course problem 1 I'm solving right now