How do you make functions that take a variable that is not defined?

[snippsat]

That's not default argument. I know what default argument is.
Those variables were never defined but passed down into the function in capital letters.

This how Tkinter dos it,you should mention where code comes from to get better answers.

>>> from tkinter import LEFT, TOP, X, FLAT, RAISED
>>> TOP
'top'
>>> LEFT
'left'
>>> FLAT
'flat'
>>> X
'x'

So toolbar.pack(side=TOP, fill=X) is the same as toolbar.pack(side='top', fill='x')
So it could work fine as default argument to a function.

With toolbar.pack() they are passed in as key word arguments(**kwargs).
If look at method parameters in Tkinter code.

def pack_configure(self, cnf={}, **kw):

Example:

>>> def my_function(**kw):
...     print(kwargs)  
...     
>>> my_function(a=123, b="hello")
{'a': 123, 'b': 'hello'}