Using default MTU value warning when using client.mtu_size()

[ChrisM0815]

• bleak version: 0.19.5
• Python version: 3.10.8
• Operating System: Linux
• BlueZ version (bluetoothctl -v) in case of Linux: 5.55

Description

When I call client.mtu_size(self) I´m getting the following warning on a Raspberry Pi:
bleak/backends/bluezdbus/client.py:561: UserWarning: Using default MTU value. Call _acquire_mtu() or set _mtu_size first to avoid this warning.

What I Did

import asyncio
import sys

from bleak import BleakScanner, BleakClient
from bleak.backends.winrt.client import logger as bleak_log

DEFAULT_MAC = "05:55:F5:73:C5:E7"
async def main(ble_address: str):
    print("Connecting")
    device = await BleakScanner.find_device_by_address(ble_address, timeout=20.0)
 
    async with BleakClient(device) as client:
        print(client.mtu_size)
        
if __name__ == "__main__":
    asyncio.run(main(sys.argv[1] if len(sys.argv) == 2 else DEFAULT_MAC))

I´m using this Script to connect to a ble device and print out the MTU. On Linux I ´m getting the above mentioned warning. I was not able to call the client._acquire_mtu() method. Is there a way to get a higher MTU on Linux or at least prevent this warning from printing?
Thanks

[dlech]

If possible, you should use BleakGATTCharacteristic.max_write_without_response_size instead of BleakClient.mtu_size.

To avoid the warning, call client._backend._mtu_size = EXPECTED_MTU before reading client.mtu_size.

[EdwinFairchild]

is Apple macOS able to get MTU from client.mtu_size ? I have no way of testing.

[dlech]

It is recommended to use https://bleak.readthedocs.io/en/latest/api/index.html#bleak.backends.characteristic.BleakGATTCharacteristic.max_write_without_response_size in new code instead. Both work on macOS.

[EdwinFairchild]

yeah I saw that , but I am trying to get MTU size of a server that will request a smaller one and the characteristics I am using are not write without response, I act based on the response.

[EdwinFairchild]

Also saw the work around in mtu_size.py but inspite of being on Ubuntu the if statement fails, I used other methods to test if I am linux and I am setting a smaller MTU evne though I cannot really get the actual MTU size on linux. On windows it works.

[dlech]

It doesn't matter what the characteristic supports it will still return the same value. max_write_without_response_size is still the best way to get the value cross-platform otherwise you have to provide your own implementation for Linux. If you really need MTU, you can just add 3 to this value.