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

[Alan Gauld]

On 14/08/15 05:31, boB Stepp wrote:
> 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.

This is a rubbish example, or at the kindest an incomplete one.
The problem here is that the class about() method is relying on the 
existence of global variables called bill and tail.
In any sane example it should be using the attributes self.bill and 
self.tail.

If they really did want all instances to share the same bill and tail 
they should have made them class variables rather than instance ones and 
used those in about().

So, in normal use of composition you, Bob, would be right and they 
should use self.bill and self.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.

This is because of the hard coded references to the global
vars bill and tail. If they used self.bill and self.tail
everything would work as you expected.

So, unless the book explains why this is bad practice and
goes on to show a good example, I must conclude its a very
bad example.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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