Troubleshooting

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

Common Mistakes

Calling asyncio.run() more than once

Bleak requires the same asyncio run loop to be used for all of its operations. And it requires the loop to always be running because there are background tasks that need to always be running. Therefore, make sure you only call asyncio.run() once at the start of your program. Your program will not work correctly if you call it more than once. Even if it seems like it is working, crashes and other problems will occur eventually.

DON’T!

async def scan():
    return await BleakScanner.find_device_by_name("My Device")

async def connect(device):
    async with BleakClient(device) as client:
        data = await client.read_gatt_char(MY_CHAR_UUID)
        print("received:" data)

# Do not wrap each function call in asyncio.run() like this!
device = asyncio.run(scan())
if not device:
    print("Device not found")
else:
    asyncio.run(connect(device))

DO!

async def scan():
    return await BleakScanner.find_device_by_name("My Device")

async def connect(device):
    async with BleakClient(device) as client:
        data = await client.read_gatt_char(MY_CHAR_UUID)
        print("received:" data)

# Do have one async main function that does everything.
async def main():
    device = await scan()
    if not device:
        print("Device not found")
        return

    await connect(device)

asyncio.run(main())

DON’T!

async def scan_and_connect():
    device = await BleakScanner.find_device_by_name("My Device")
    if not device:
        print("Device not found")
        return

    async with BleakClient(device) as client:
        data = await client.read_gatt_char(MY_CHAR_UUID)
        print("received:" data)


while True:
    # Don't call asyncio.run() multiple times like this!
    asyncio.run(scan_and_connect())
    # Never use blocking sleep in an asyncio programs!
    time.sleep(5)

DO!

async def scan_and_connect():
    device = await BleakScanner.find_device_by_name("My Device")
    if not device:
        print("Device not found")
        return

    async with BleakClient(device) as client:
        data = await client.read_gatt_char(MY_CHAR_UUID)
        print("received:" data)

# Do have one async main function that does everything.
async def main():
    while True:
        await scan_and_connect()
        # Do use asyncio.sleep() in an asyncio program.
        await asyncio.sleep(5)

asyncio.run(main())

Naming your script bleak.py

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

If the app is already in the list but the checkbox for Bluetooth is disabled, you will get the a BleakError: “BLE is not authorized - check macOS privacy settings”. instead of crashing with SIGABRT, in which case you need to check the box to allow Bluetooth for the app that is running Python.

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.

Windows Bugs

Not working when threading model is STA

Packages like pywin32 and it’s subsidiaries have an unfortunate side effect of initializing the threading model to Single Threaded Apartment (STA) when imported. This causes async WinRT functions to never complete if Bleak is being used in a console application (no Windows graphical user interface). This is because there isn’t a Windows message loop running to handle async callbacks. Bleak, when used in a console application, needs to run in a Multi Threaded Apartment (MTA) instead (this happens automatically on the first WinRT call).

Bleak should detect this and raise an exception with a message similar to:

Thread is configured for Windows GUI but callbacks are not working.

You can tell a pywin32 package caused the issue by checking for "pythoncom" in sys.modules. If it is there, then likely it triggered the problem. You can avoid this by setting sys.coinit_flags = 0 before importing any package that indirectly imports pythoncom. This will cause pythoncom to use the default threading model (MTA) instead of STA.

Example:

import sys
sys.coinit_flags = 0  # 0 means MTA

import win32com  # or any other package that causes the issue

If the issue was caused by something other than the pythoncom module, there are a couple of other helper functions you can try.

If your program has a graphical user interface and the UI framework and it is properly integrated with asyncio and Bleak is not running on a background thread then call allow_sta() before calling any other Bleak APis:

try:
    from bleak.backends.winrt.util import allow_sta
    # tell Bleak we are using a graphical user interface that has been properly
    # configured to work with asyncio
    allow_sta()
except ImportError:
    # other OSes and older versions of Bleak will raise ImportError which we
    # can safely ignore
    pass

The more typical case, though, is that some library has imported something similar to pythoncom with the same unwanted side effect of initializing the main thread of a console application to STA. In this case, you can uninitialize the threading model like this:

import naughty_module  # this sets current thread to STA :-(

try:
    from bleak.backends.winrt.util import uninitialize_sta

    uninitialize_sta()  # undo the unwanted side effect
except ImportError:
    # not Windows, so no problem
    pass

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():
    devices: Sequence[BLEDevice] = await BleakScanner.discover(timeout=5.0)

    for d in devices:
        async with BleakClient(d) as client:
            print(client.services)


asyncio.run(find_all_devices_services())

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.