Why is this?

Duncan Booth duncan.booth at invalid.invalid
Fri Aug 12 14:29:58 CEST 2005


Peter Mott 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?
> 
What you did was not S+S. You did S+T, i.e. you added two distinct lists. 
Try it again adding the same list and you will see that both addition and 
multplication do work the same:

>>> S = [[]]
>>> L = S + S
>>> L[1].append(1)
>>> L
[[1], [1]]
>>> S = [[]]
>>> L = 2*S
>>> L[1].append(1)
>>> L
[[1], [1]]



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