I added type hints to the asyncio example titled TCP echo server using streams from the official documentation. Mypy wrongly complains that Value of type "Optional[List[socket]]" is not indexable pointing at line 23, which is:

    addr = server.sockets[0].getsockname()

Quick check:

• 3.6.10: simple attribute, can be None
• 3.7.10: property, returns list, never None
• 3.8.3: property, returns tuple, never None

Also, socket is defined on AbstractServer in typeshed, not on Server. On all checked versions, AbstractServer doesn't defined socket at all.

It's worth noting that asyncio.trsock.TransportSocket is explicitly not a subtype of socket.socket. Its docstring states:

All potentially disruptive operations (like "socket.close()") are banned.

Thank you @srittau for addressing this so quickly!

I saw you annotated the sockets attribute as Iterable.

However, both examples that I mentioned from the standard library use it as a Sequence like this:

    addr = server.sockets[0].getsockname()

The examples are:

• example code in the asyncio docs;
• test code in Lib/test/test_asyncio/test_server.py.

@ramalho The attributes/properties are annotated as list or Tuple. Only the argument on 3.7+ is annotated as Iterable and that should work, as the implementation uses list(self._sockets) or tuple(self._sockets).

I am sorry but don't understand your answer, @srittau. Yes, the annotation is compatible with the implementations, but the Iterable type is too general.

The documentation for asyncio.Server.sockets in Python 3.7 and later states that the attribute returns a list.

Changed in version 3.7: Prior to Python 3.7 Server.sockets used to return an internal list of server sockets directly. In 3.7 a copy of that list is returned.

Users expect to be able to index a list, and both examples from CPython itself (docs and tests) do this:

    addr = server.sockets[0].getsockname()

Therefore, the Iterable annotation will cause Mypy to flag user's code that uses sockets as a Sequence according to the Python ≥ 3.7 documentation and examples provided in the distribution.

But the sockets attribute isn't annotated with Iterable. It's annotated with (depending on the version) Optional[list[_socket]], list[_socket], or Tuple[_socket, ...].

FYI, the code sample from the asyncio docs still does not type check because asyncio.create_server returns an instance of AbstractServer which does not implement sockets, and you can't even use an assertion prior to accessing sockets anymore.

assert isinstance(server, Server) should work, right?

Also, should we make create_server() return a Server instead of an AbstractServer in the stub?

assert isinstance(server, Server) should work, right?

That works but Server is not exposed in __init__ and would have to be imported from base_events. It's a bit much for a type-only assertion.

Also, should we make create_server() return a Server instead of an AbstractServer in the stub?

That'd fix it for the top-level start_server function. asyncio.get_{event,running}_loop() would have to return a BaseEventLoop for the loop's create_server method to return a Server as well.

Cf. #5566. loop.create_server() is documented to return Server, not AbstractServer anyway.

Great, thanks!