"the type initializer for Delegates threw an exception" - using Create Runtime

[vruetten]

Hi all
I'm a newbie to Bonsai and am trying to get the "Create Runtime" node working.

I've tried following the documentation: https://bonsai-rx.org/python-scripting/?tabs=windows
but still run into an error.

I've installed python (v3.12.8). I created an environment as instructed via python -m venv .venv, activated it and tested that I could indeed launch python from the command line using this environment.
I then point the runtime node to this folder (with or without an empty script in the "ScriptPath") with or without the python environment activated and get the "Delegate" error.

Any ideas what I might be doing wrong?
Thanks in advance for any help.

I'm running

• Windows 11 Pro
• Bonsai 2.8.5
• Python 3.12.8

Prior to this I attempted installing python via miniforge (and tried various versions of python) but couldn't get past this error.

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[ncguilbeault]

Hi @vruetten ,

It's cool to see you are trying out the Bonsai - Python scripting package! In my experience, the “Delegate” error in Bonsai often means that Bonsai can’t locate or load the Python DLL correctly. This is frequently caused by issues in the system PATH or conflicts between multiple Python installations. Below are some steps I recommend to help get started:

Check for existing Python paths

Open a Command Prompt (or PowerShell) and type:

where python

This should list all instances of python.exe that your system sees. If there’s more than one, it's possible that this can create problems when Bonsai tries to load the DLL.

Uninstall / remove conflicting Python installations

Bonsai will always try to load the first Python installation found on the PATH. So, if you see multiple Python installations (e.g., from the Microsoft Store, Miniconda, Miniforge), it might be helpful to uninstall them so you have only one “clean” Python installation, or prioritize the clean installation by making sure it appears first in the system PATH. You should verify that the Python installation is correct by typing:

where python
python --version

where python should contain the Python installation you want to use and python --version should print out the correct version.

Installing a fresh copy of Python

If you haven't already, download the “Windows installer (64-bit)” for the version you want from python.org/downloads/windows (I use python 3.10.12 but python 3.12.8 has been tested and also works). During installation, check the option to “Add python.exe to PATH” so that Bonsai can detect it correctly. It may also be useful to select the Disable path length limit option if prompted.
After installing, restart your PC - this ensures that any changes to the system PATH will take effect.

Test in Bonsai

Once you have your clean Python installation ready, you can test it in Bonsai first without using a virtual environment. For this, simply open Bonsai and create a new workflow. Add a CreateRuntime node and run the workflow. If everything is configured correctly, the workflow should run without any errors. An important thing to note about virtual environments: if you create a new venv while another virtual environment is active, it won't work correctly. You should confirm that the home variable in your .venv\pyvenv.cfg file points to your clean Python installation (for example, home = C:\Users\[YourUsername]\AppData\Local\Programs\Python\Python312). If it points to a different location, like a conda environment or another virtual environment, then this will likely cause issues.

Another important thing to note: Bonsai uses Python.NET under the hood, so the Python engine is tied to the Bonsai application process, rather than each individual workflow run. If Python fails to load on the first attempt, you may need to restart the Bonsai application entirely before making changes and trying again. An easy way to do this is to press the Reload Extensions button in the toolbar.

Hopefully these steps can help you resolve the “Delegates” error. If you keep seeing the error or have additional questions, let me know and we can troubleshoot further. Good luck, and I hope this helps!

[vruetten]

worked a charm - thanks!