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

[magic]

It's called Default Argument
Example:

def foo(x, y=5):
    print(f'The sum is: {sum((x, y))}')

Now if i use this function i only need to give argument for x parameter.
y will always be 5 until give two arguments,then default value get overwritten.

>>> foo(4)
The sum is: 9
>>> foo(10)
The sum is: 15
>>> 
>>> foo(7, 7)
The sum is: 14
>>> foo(100, 599)
The sum is: 699

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.