After a ttk.Button has been created, the name of the callback function can be obtained in various ways.  But is there a way to obtain the callback function itself?
The use case is that I want to use the button's callback function in the callback function to be assigned to another widget.
Currently I'm assigning the button's callback function to an additional attribute of the Button object after creating it.  That's redundant, though, and I'd like to get the callback function directly from the Button object itself, if possible.
No, there is no supported way to get the command via introspection. Your solution of adding it as an attribute is a perfectly reasonable thing to do. If you're bothered by the fact it takes two lines of code instead of one, you can subclass the button to hide that detail.
How can the command function be obtained from a Tkinter button?