[Tutor] Does composition only work with particular instances of objects?

[boB Stepp]

I was looking at an example illustrating composition from the book,
"Introducing Python" by Bill Lubanovic on p. 140:

>>> class Bill:
        def __init__(self, description):
            self.description = description

>>> class Tail:
        def __init__(self, length):
            self.length = length

>>> class Duck:
        def __init__(self, bill, tail):
            self.bill = bill
            self.tail = tail
        def about(self):
            print('This duck has a', bill.description, 'bill and a',
                  tail.length, 'tail.')

Here I was mildly surprised that bill and tail were not Bill and Tail,
and in the about method that self.bill was not used in place of
bill.description, etc.

Continuing:

>>> tail = Tail('long')
>>> bill = Bill('wide orange')
>>> duck = Duck(bill, tail)
>>> duck.about()
This duck has a wide orange bill and a long tail.

So I naively thought I could do the following:

>>> bill0 = Bill('narrow rainbow')
>>> tail0 = Tail('ginormous')

And was surprised by:

>>> duck.about()
This duck has a wide orange bill and a long tail.
>>> duck0 = Duck(bill0, tail0)
>>> duck0.about()
This duck has a wide orange bill and a long tail.

>From this I am forced to conclude that composition will only work with
particular instances of objects and not with any old objects created
from their respective classes.  Is this understanding correct?

Thanks!

-- 
boB