Why is this?

[Peter Mott]

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?

Peter



The excellent Martelli Python Nutshell says that multiplication S*n is 
"the concatenation of n copies of S" so S+S and S*2 would be expected to 
be the same. It seems though that they are not but that * creates a list


Jiri Barton wrote:
> Hi everyone,
> 
> I have a problem with initialization.
> 
>>>>a, b = [[]]*2
>>>>a.append(1)
>>>>b
> 
> [1]
> 
> Why is this? Why does not this behave like the below:
> 
> 
>>>>a, b = [[], []]
>>>>a.append(1)
>>>>b
> 
> []
> 
> And, just to add to my confusion:
> 
> 
>>>>[[]]*2
> 
> [[], []]
> 
>>>>[[], []] == [[]]*2
> 
> True
> 
> Thanks in advance for the explanation.
> jbar
> 
> 
> BTW, if it matters...
> Python 2.4.1 (#2, Mar 30 2005, 20:41:35)
> [GCC 3.3.5 (Debian 1:3.3.5-8ubuntu2)] on linux2
> 
> 
>