Debug Kusto Query Language inline Python using VS Code

Azure Data Explorer supports running Python code embedded in Kusto Query Language using the python() plugin. The plugin runtime is hosted in a sandbox, an isolated and secure Python environment. The python() plugin capability extends Kusto Query Language native functionalities with the huge archive of OSS Python packages. This extension enables you to run advanced algorithms, such as machine learning, artificial intelligence, statistical, and time series as part of the query.

Kusto Query Language tools aren't convenient for developing and debugging Python algorithms. Therefore, develop the algorithm on your favorite Python-integrated development environment such as Jupyter, PyCharm, VS, or VS Code. When the algorithm is complete, copy and paste into KQL. To improve and streamline this workflow, Azure Data Explorer supports integration between Kusto Explorer or Web UI clients and VS Code for authoring and debugging KQL inline Python code.


This workflow can only be used to debug relatively small input tables (up to few MB). Therefore, you may need to limit the input for debugging. If you need to process a large table, limit it for debugging using | take, | sample, or where rand() < 0.x.


Run your query in your client application

  1. In your client application, prefix a query containing inline Python with set query_python_debug;

  2. Run the query.

    • Kusto Explorer: VS Code is automatically launched with the script.
    • Kusto Web UI:
      1. Download and save, df.txt, and kargs.txt. In window, select Allow. Save files in selected directory.

        Web UI downloads the inline python files.

      2. Right-click and open with VS Code. The script contains the inline Python code, from the KQL query, prefixed by the template code to initialize the input dataframe from df.txt and the dictionary of parameters from kargs.txt.

  3. In VS Code, launch the VS Code debugger: Debug > Start Debugging (F5), select Python configuration. The debugger will launch and automatically breakpoint to debug the inline code.

How does inline Python debugging in VS Code work?

  1. The query is parsed and executed in the server until the required | evaluate python() clause is reached.
  2. The Python sandbox is invoked but instead of running the code, it serializes the input table, the dictionary of parameters, and the code, and sends them back to the client.
  3. These three objects are saved in three files: df.txt, kargs.txt, and in the selected directory (Web UI) or in the client %TEMP% directory (Kusto Explorer).
  4. VS Code is launched, preloaded with the file that contains a prefix code to initialize df and kargs from their respective files, followed by the Python script embedded in the KQL query.

Query example

  1. Run the following KQL query in your client application:

    range x from 1 to 4 step 1
    | evaluate python(typeof(*, x4:int), 
    'exp = kargs["exp"]\n'
    'result = df\n'
    'result["x4"] = df["x"].pow(exp)\n'
    , bag_pack('exp', 4))

    See the resulting table:

    x x4
    1 1
    2 16
    3 81
    4 256
  2. Run the same KQL query in your client application using set query_python_debug;:

    set query_python_debug;
    range x from 1 to 4 step 1
    | evaluate python(typeof(*, x4:int), 
    'exp = kargs["exp"]\n'
    'result = df\n'
    'result["x4"] = df["x"].pow(exp)\n'
    , bag_pack('exp', 4))
  3. VS Code is launched:

    launch VS Code.

  4. VS Code debugs and prints 'result' dataframe in the debug console:

    VS Code debug.


There may be differences between the Python sandbox image and your local installation. Check the sandbox image for specific packages by querying the plugin.


In case there are errors when launching VS code debugging session try setting the default terminal profile to command prompt:

  • Press CTRL + SHIFT + P to open the Command Palette.
  • Search for "Terminal: Select Default Profile" and select "Command Prompt"