AVPython allows you to use the popular Python language, and all of its extensions, to customize ArcView GIS.
To call Python code from inside ArcView, just load the "Python Language Support" extension into your project, and use the Python.Exec or Python.Eval scripts that are then available. To call back to ArcView from Python, just say "from arcview import avexec" in your Python script, and then use the avexec function to run Avenue code. However, you can only do that from scripts that are running inside of ArcView (i.e. using the "Python Language Support" extension).