[ [ [] * y for y in range(3) ] * x for x in range(2) ]

Andrew Bennetts andrew-pythonlist at puzzling.org
Thu Jan 30 06:09:42 CET 2003


On Wed, Jan 29, 2003 at 08:34:05PM -0800, Marcel wrote:
> Hi,
> 
> I'm trying to allocate nested lists up-front using the subject line,
> instead of generating :
> [ [ [], [], [] ], [ [], [], [] ] ]
> I end up with  :
> [ [], [ [], [], [] ] ]
> 
> I guess I screwed up something, but I'm failing to understand what...
> 
> Any idea ?

Try:
    [[[] for y in range(3)] for x in range(2)]

You don't want to multiply the lists, which copies the contents -- in fact,
multiplying an empty list by anything will always give you an empty list.

Why are you pre-allocating the lists, anyway?  The speed difference should
be negligible to just doing someList.append(..) as appropriate.

-Andrew.






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