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Note: This article describes a deprecated feature as of Panda3D 1.10.0.
AppRunner is the Python class that supervises the launching of your application in a p3d file. There is a global instance of AppRunner available in base.appRunner whenever your application is running as a p3d file. In fact, this is a reliable way to test your run mode:
if base.appRunner: print("Running in a p3d file") else: print("Running interactively")