Why is this?

[Matt Hammond]

On Fri, 12 Aug 2005 12:57:38 +0100, Peter Mott <peter at monicol.co.uk> wrote:

> If I use concatenation + instead of multiplication * then I get the  
> result that Jiri expected:
>
>  >>> L = [[]] + [[]]
>  >>> L[1].append(1)
>  >>> L
> [[], [1]]
>
> With * both elements are changed:
>
>  >>> L = [[]] * 2
>  >>> L[1].append(1)
>  >>> L
> [[1], [1]]
>
> Alex Martelli says in his excellent Nutshell book that + is  
> concatenation and that "n*S is the concatenation of n copies of S". But  
> it seems not so. Surely, from a logical point of view, S + S should be  
> the same as S * 2?

S+S is the same as S*2, but L= [[]] + [[]] is not S+S. The two terms being  
added are different instances of an empty list. You are  
adding/concatenating two different object instances.

Suppose I do concatenate two of the same object instance, then I get the  
same behaviour as with the multiply example:
>>> T = [[]]
>>> L = T + T
>>> L[1].append(1)
>>> L
[[1], [1]]

In fact, you could argue this is exactly what the multiply operation is  
doing. (internally the implementation may be slightly different, but it is  
still equivalent to this)

regards


Matt
-- 

| Matt Hammond
| R&D Engineer, BBC Research and Development, Tadworth, Surrey, UK.