I've done some more research and this is an integration issue between pyglet and Tk. In particular:

* Both pyglet and Tk use AppKit to implement their GUI
* AppKit uses an NSApplication class, and in particular a singleton instance of that class, to represent the application, which will get instantiated during application startup
* The Tk library uses a sublclass of NSApplication with additional functionality (TkApplication).
* When pyglet is started first the NSApplication singleton is an instance of NSApplication and not of TkApplication, but Tk's implementation assumes that the singleton is an instance of TkApplicationo.

A workaround for this issue is to make sure Tk is initialised before initialising pyglet. For example using this code at the start of the script: "from tkinter import Tk; root = Tk()"


There's not much we can do about this in CPython, other than maybe filing an issue with the Tk project about this. I haven't looked at the Tk codebase and there cannot tell how hard it would be for then to avoid a dependency on TkApplication as a separate class (for example by implementing the additional functionality they need in a category on NSApplication).