On 5 November 2012 09:13, Hans Mulder <han...@xs4all.nl> wrote: > On 5/11/12 07:27:52, Demian Brecht wrote: >> So, here I was thinking "oh, this is a nice, easy way to initialize a 4D >> matrix" >> (running 2.7.3, non-core libs not allowed): >> >> m = [[None] * 4] * 4 >> >> The way to get what I was after was: >> >> m = [[None] * 4, [None] * 4, [None] * 4, [None * 4]] > > Or alternateively: > > m = [[None] * 4 for _ in range(4)]
That's the way to do it. I've seen this question many times between here and the python-tutor list. It does seem to be a common gotcha. I was just thinking to myself that it would be a hard thing to change because the list would need to know how to instantiate copies of all the different types of the elements in the list. Then I realised it doesn't. It is simply a case of how the list multiplication operator is implemented and whether it chooses to use a reference to the same list or make a copy of that list. Since all of this is implemented within the same list type it is a relatively easy change to make (ignoring backward compatibility concerns). I don't see this non-copying list multiplication behaviour as contradictory but has anyone ever actually found a use for it? Oscar -- http://mail.python.org/mailman/listinfo/python-list
