Troubleshooting

When things don’t seem to be working right, here are some things to try.

Common Mistakes

Many people name their first script `bleak.py. This causes the script to crash with an ImportError similar to:

ImportError: cannot import name 'BleakClient' from partially initialized module 'bleak' (most likely due to a circular import) (bleak.py)`

To fix the error, change the name of the script to something other than bleak.py.

macOS Bugs

Bleak crashes with SIGABRT on macOS

If you see a crash similar to this:

Crashed Thread:        1  Dispatch queue: com.apple.root.default-qos

Exception Type:        EXC_CRASH (SIGABRT)
Exception Codes:       0x0000000000000000, 0x0000000000000000
Exception Note:        EXC_CORPSE_NOTIFY

Termination Reason:    Namespace TCC, Code 0
This app has crashed because it attempted to access privacy-sensitive data without a usage description. The app's Info.plist must contain an NSBluetoothAlwaysUsageDescription key with a string value explaining to the user how the app uses this data.

It is not a problem with Bleak. It is a problem with your terminal application.

Ideally, the terminal application should be fixed by adding NSBluetoothAlwaysUsageDescription to the Info.plist file (example).

It is also possible to manually add the app to the list of Bluetooth apps in the Privacy settings in the macOS System Preferences.

_images/macos-privacy-bluetooth.png

No devices found when scanning on macOS 12

A bug was introduced in macOS 12.0 that causes scanning to not work unless a list of service UUIDs is provided to BleakScanner. This bug was fixed in macOS 12.3. On the affected version, users of bleak will see the following error logged:

macOS 12.0, 12.1 and 12.2 require non-empty service_uuids kwarg, otherwise no advertisement data will be received

See #635 and #720 for more information including some partial workarounds if you need to support these macOS versions.

Enable Logging

The easiest way to enable logging is to set the BLEAK_LOGGING environment variable. Setting the variable depends on what type of terminal you are using.

Posix (Linux, macOS, Cygwin, etc.):

export BLEAK_LOGGING=1

Power Shell:

$env:BLEAK_LOGGING=1

Windows Command Prompt:

set BLEAK_LOGGING=1

Then run your Python script in the same terminal.

Connecting to multiple devices at the same time

If you’re having difficulty connecting to multiple devices, try to do a scan first and pass the returned BLEDevice objects to BleakClient calls.

Python:

import asyncio
from typing import Sequence

from bleak import BleakClient, BleakScanner
from bleak.backends.device import BLEDevice


async def find_all_devices_services()
    scanner = BleakScanner()
    devices: Sequence[BLEDevice] = scanner.discover(timeout=5.0)
    for d in devices:
        async with BleakClient(d) as client:
            print(await client.get_services())


asyncio.run(find_all_devices_services())

Pass more parameters to a notify callback

If you need a way to pass more parameters to the notify callback, please use functools.partial to pass in more arguments.

Issue #759 might fix this in the future.

Python:

from functools import partial

from bleak import BleakClient


def my_notification_callback_with_client_input(
    client: BleakClient, sender: int, data: bytearray
):
    """Notification callback with client awareness"""
    print(
        f"Notification from device with address {client.address} and characteristic with handle {client.services.get_characteristic(sender)}. Data: {data}"
    )

# [...]

await client.start_notify(
    char_specifier, partial(my_notification_callback_with_client_input, client)
)

Capture Bluetooth Traffic

Sometimes it can be helpful to see what is actually going over the air between the OS and the Bluetooth device. There are tools available to capture HCI packets and decode them.

Windows 10

There is a Windows hardware developer package that includes a tool that supports capturing Bluetooth traffic directly in Wireshark.

Install

  1. Download and install Wireshark.
  2. Download and install the BTP software package.

Capture

To capture Bluetooth traffic:

  1. Open a terminal as Administrator.

    • Search start menu for cmd. (Powershell and Windows Terminal are fine too.)

    • Right-click Command Prompt and select Run as Administrator.

      Screenshot of Windows Start Menu showing Command Prompt selected and context menu with Run as Administrator selected.
  2. Run C:\BTP\v1.9.0\x86\btvs.exe. This should automatically start Wireshark in capture mode.

    Tip

    The version needs to match the installed version. v1.9.0 was the current version at the time this was written. Additionally, C: may not be the root drive on some systems.

  3. Run your Python script in a different terminal (not as Administrator) to reproduce the problem.

  4. Click the stop button in Wireshark to stop the capture.

macOS

On macOS, special software is required to capture and view Bluetooth traffic. You will need to sign up for an Apple Developer account to obtain this software.

  1. Go to https://developer.apple.com/download/more/ and download Additional Tools for Xcode … where … is the Xcode version corresponding to your macOS version (e.g. 12 for Big Sur, 11 for Mojave, etc.).
  2. Open the disk image and in the Hardware folder, double-click the PacketLogger.app to run it.
  3. Click the Clear button in the toolbar to clear the old data.
  4. Run your Python script to reproduce the problem.
  5. Click the Stop button in the toolbar to stop the capture.

Tip

The Bluetooth traffic can be viewed in the PacketLogger.app or it can be saved to a file and viewed in Wireshark.

Linux

On Linux, Wireshark can be used to capture and view Bluetooth traffic.

  1. Install Wireshark. Most distributions include a wireshark package. For example, on Debian/Ubuntu based distributions:

    sudo apt update && sudo apt install wireshark
    
  2. Start Wireshark and select your Bluetooth adapter, then start a capture.

    Tip

    Visit the Wireshark Wiki for help with configuring permissions and making sure proper drivers are installed.

  3. Run your Python script to reproduce the problem.

  4. Click the stop button in Wireshark to stop the capture.

Handling OS Caching of BLE Device Services

If you develop your own BLE peripherals, and frequently change services, characteristics and/or descriptors, then Bleak might report outdated versions of your peripheral’s services due to OS level caching. The caching is done to speed up the connections with peripherals where services do not change and is enabled by default on most operating systems and thus also in Bleak.

There are ways to avoid this on different backends though, and if you experience these kinds of problems, the steps below might help you to circumvent the caches.

macOS

The OS level caching handling on macOS has not been explored yet.

Linux

When you change the structure of services/characteristics on a device, you have to remove the device from BlueZ so that it will read everything again. Otherwise BlueZ gives the cached values from the first time the device was connected. You can use the bluetoothctl command line tool to do this:

bluetoothctl -- remove XX:XX:XX:XX:XX:XX
# prior to BlueZ 5.62 you also need to manually delete the GATT cache
sudo rm "/var/lib/bluetooth/YY:YY:YY:YY:YY:YY/cache/XX:XX:XX:XX:XX:XX"

…where XX:XX:XX:XX:XX:XX is the Bluetooth address of your device and YY:YY:YY:YY:YY:YY is the Bluetooth address of the Bluetooth adapter on your computer.