Attempting to access BleakClient.mtu_size triggers warning about _acquire_mtu

[fixermark]

• bleak version: 0.20.1
• Python version: 3.9.12
• Operating System: Ubuntu 5.4.0-144-generic
• BlueZ version (bluetoothctl -v) in case of Linux: bluetoothctl: 5.53

Description

After creating a BleakClient and calling client.connect(), I attempt to access client.mtu_size.

Expected result:
mtu_size retrieved.

Observed result:
A warning is emitted:

<$USER>/miniconda3/lib/python3.9/site-packages/bleak/backends/bluezdbus/client.py:620: UserWarning: Using default MTU value. Call _acquire_mtu() or set _mtu_size first to avoid this warning.        
  warnings.warn(

The mtu_size shows as 23. Monocle's preferred MTU is 128.

Even though it is single-underscore-prefixed, I also attempted to call _acquire_mtu first, but the method does not appear to exist on BleakClient. Scanning the source code tells me it is a method on BluezDBus client however, but it's unclear to me how to get to that method from BleakClient (and I suspect I'm not supposed to be calling it directly...).

What I Did

I'm writing a small client for the Brilliant Labs Monocle, which has a MicroPython shell accessible via UART.

Minimum workable example:

import asyncio
from bleak import BleakClient

async def example():
  address = '<address of Monocle>'
  client = BleakClient(address)
  await client.connect()
  print(client.mtu_size)

asyncio.run(example())

I am unaware of anything Monocle-specific that would prevent reproduction of this phenomenon on other devices (but a Monocle is all I have access to at this time).

Logs

Logs attached.
out.log

[dlech]

This is the documented behavior: https://bleak.readthedocs.io/en/latest/api/client.html#bleak.BleakClient.mtu_size

Please use BleakGATTCharacteristic.max_write_without_response_size instead as suggested or provide your own implementation as in https://github.com/hbldh/bleak/blob/develop/examples/mtu_size.py. You can hard-code 128 as the MTU instead of using _acquire_mtu if you are sure it will always be correct.

[fixermark]

It appears max_write_without_response_size on Monocle's RX characteristic is also returning 20, which may be an issue with Monocle. We'll hard-code a value in our application. Thank you for the help!

[dlech]

Actually the 20 can be because BlueZ is older than v5.62. But hard-coding in that case is still the appropriate workaround.