On Fri, 12 Aug 2005 12:57:38 +0100, Peter Mott <[EMAIL PROTECTED]> 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. -- http://mail.python.org/mailman/listinfo/python-list
