assistant: Here's an updated version with:
 
Reduced redundancy in comments
Added docstrings
Added type hints
```py 
import os
import asyncio
import datetime as dt
 
from logger import get_logger
from nio import AsyncClient, MatrixRoom, RoomMessageText
 
_logger = get_logger()  # type: ignore
 
ELEM_ROOM = os.environ.get("ELEM_ROOM")  # type: str
ELEM_USR = os.environ.get("ELEM_USR")  # type: str
ELEM_PSW = os.environ.get("ELEM_PSW")  # type: str
 
def send(msg: str) -> None:
    """Send a message to a Matrix room."""
 
    async def message_callback_send(room: MatrixRoom, event: RoomMessageText) -> None:
        """Log sent messages."""
        _logger.info(f"Message sent to room {room.display_name} | "
                     f"{room.user_name(event.sender)}: {event.body}")
 
    async def send_message() -> None:
        """Connect to Matrix and send a message."""
        client = AsyncClient("https://matrix.org", ELEM_USR)
        client.add_event_callback(message_callback_send, RoomMessageText)
        try:
            _logger.info(await client.login(ELEM_PSW))
        except ValueError as e:
            _logger.error(f"Failed to login - wrong or missing password?\n{e}")
            raise e
        await client.room_send(
            room_id=ELEM_ROOM,
            message_type="m.room.message",
            content={"msgtype": "m.text", "body": msg},
        )
        _logger.info(f"Message sent to room {ELEM_ROOM} | {ELEM_USR}: {msg}")
        await client.close()
 
    asyncio.get_event_loop().run_until_complete(send_message())
 
if __name__ == "__main__":
    msg = f"Hello world! {dt.datetime.now()}"
    send(msg)
```

assistant: I ran an auto-formatter and a linter on it. Rather than comments, I added docstrings, which can be seen more easily by IDEs and documentation generators like Sphinx.

```
"""Implementation and example usage for a Matrix message sending function."""
# https://matrix-nio.readthedocs.io/en/latest/examples.html#a-basic-client

from logger import get_logger
from nio import AsyncClient, MatrixRoom, RoomMessageText
import os
import asyncio
import datetime as dt

_logger = get_logger()

ELEM_ROOM = os.environ.get("ELEM_ROOM")
ELEM_USR = os.environ.get("ELEM_USR")
ELEM_PSW = os.environ.get("ELEM_PSW")


def send(message: str) -> None:
    """Send message to the Matrix room with environment variable ELEM_ROOM from ELEM_USR user with ELEM_PSW password."""

    async def message_callback_send(room: MatrixRoom, event: RoomMessageText) -> None:
        _logger.info(
            f"Message sent to room {room.display_name} | {room.user_name(event.sender)}: {event.body}"
        )

    async def send_message() -> None:
        client = AsyncClient("https://matrix.org", ELEM_USR)
        client.add_event_callback(message_callback_send, RoomMessageText)
        try:
            _logger.info(await client.login(ELEM_PSW))
        except ValueError as e:
            _logger.error(f"Failed to login - wrong or missing password?\n{e}")
            raise e
        await client.room_send(
            room_id=ELEM_ROOM,
            message_type="m.room.message",
            content={"msgtype": "m.text", "body": message},
        )
        _logger.info(f"Message sent to room {ELEM_ROOM} | {ELEM_USR}: {message}")
        await client.close()

    asyncio.get_event_loop().run_until_complete(send_message())


if __name__ == "__main__":
    msg = f"Hello world! {dt.datetime.now()}"
    send(msg)

```


I renamed msg to message so it didn't conflict with outer scope variables, removed commented out code (one of which wouldn't have logged any new variables anyway since the line below already logged both of them), and removed a redundant parenthesis. I also added typing to the send function, since it was already used in the internal async functions, as well as documentation matching what pydocstyle recommends.
&nbsp;

If you have access to a local matrix server, such as one through Synapse, I could also write a test to ensure the function works without disturbing other users. However, send_message would need to connect to 127.0.0.1, or wherever the local instance is, and additional code would be needed to read messages from that server.

prompter: I have access to a local matrix server. Provide me please a test for it. Tell me also about alternative testing strategies that could help and why it's recommended to use them.

assistant: Great! Here's a test for the send function:
``` 
async def test_send():
    # Set up a test room with a test user
    client = AsyncClient("https://127.0.0.1", "test_user")
    try:
        await client.login("test_password")
    except ValueError as e:
        print(f"Failed to login - wrong or missing password?\n{e}")
        raise e

    # Create a new room for testing
    test_room_id = None
    try:
        response = await client.room_create(alias="test_room")
        test_room_id = response["room_id"]
    except Exception as e:
        print(f"Failed to create test room:\n{e}")
        raise e

    # Send a message to the test room
    message = "Test message"
    await send(message)

    # Verify the message was sent
    events, end = await client.room_messages(test_room_id, limit=1)
    last_event = events[-1]
    assert last_event["content"]["body"] == message

    # Clean up the test room
    try:
        await client.room_leave(test_room_id)
        await client.room_forget(test_room_id)
    except Exception as e:
        print(f"Failed to leave or forget test room:\n{e}")
        raise e
    finally:
        await client.close()

# Run the test
asyncio.run(test_send())
``` 

Note that this test assumes a local Matrix server running on 127.0.0.1 and a test user with the credentials test_user and test_password. You'll need to adjust these values as necessary to match your setup.
\
An alternative testing strategy for this function would be to use a mocking library to create a fake AsyncClient instance and verify that its methods are called with the expected arguments. This can be useful in cases where it's difficult or time-consuming to set up a real Matrix server for testing. However, it may not catch all errors that could occur when interacting with a real Matrix server, so it's still recommended to also test the function with a real server if possible.
\
Additionally, if you're working on a larger project with many interconnected components, you may want to consider using integration testing to ensure that all the components work correctly together. Integration testing involves testing the entire system as a whole, rather than individual components in isolation. This can help catch errors that only occur when multiple components interact with each other, and can ensure that the system as a whole behaves correctly.