Connecting

To connect to a device, you first need an RfcommDeviceInfo for it. There are two ways to get one. If you already know the device's Bluetooth address, make one from it yourself, as in If you know the address. Otherwise, look the device up among the ones the system already knows, as in Looking up known devices. Then open a channel to it.

If you know the address

Make one from the device's Bluetooth address:

DEVICE = aio_rfcomm.RfcommDeviceInfo("00:16:53:11:C4:9A")

The address is six pairs of hex digits, separated by colons or by dashes. Anything else raises ValueError. Making one does not contact the device.

Looking up known devices

Pair the device in the operating system's settings before using it. Then list_known_devices lists the devices the system knows:

async with aio_rfcomm.open_bluetooth() as bluetooth:
    for device in await bluetooth.list_known_devices():
        print(device.address, device.name)

Pass a service UUID to list only the devices that offer that service:

serial_devices = await bluetooth.list_known_devices(service=SERIAL_PORT)

A device offers a service when one of its service records has that UUID as its service class. A UUID the record only mentions, such as a profile it follows, does not count. A record can name more than one class. Linux and Windows count only the first, and macOS counts each of them (seen with BlueZ 5.85, Windows 10.0.26200.9711 and macOS 26.6.2).

To find a particular device, use its address

Look for a device by its address, not by its name. The name is whatever the system has stored, and it can be out of date, missing, or wrong. The address does not change.

Opening a channel

A device offers each of its services on a numbered RFCOMM channel, from 1 to 30. There are two ways to connect.

By service UUID is the usual way. The library looks up which RFCOMM channel the device offers the service on, and connects to it:

async with bluetooth.open_service(DEVICE, SERIAL_PORT) as channel:
    ...

By channel number skips the lookup. Use it for a device whose channel number is fixed and documented. For example, the LEGO MINDSTORMS NXT uses channel 1:

async with bluetooth.open_channel(DEVICE, 1) as channel:
    ...

Channel numbers go from 1 to 30. Any other number raises ValueError. If nothing on the device is listening on that channel, the device refuses, and ServiceNotFoundError is raised.

The system keeps a stored copy of each device's services, which can be out of date. Platforms says how to refresh it on each system, and which Python builds on Linux cannot connect by number. On macOS the copy can also be empty, and Known bugs says what to do.

Next: Channels, to send and receive once connected.