Flattening lists

[Brian Allen Vanderburg II]

mrkafk at gmail.com wrote:
> Baolong zhen wrote:
>> less list creation.
>
> At the cost of doing this at each 'flatten' call:
>
> if res is None:
>    res = []
>
> The number of situations of executing above code is the same as the 
> number of list creations (once for each 'flatten' call, obviously).
>
> Is list creation really more costly than above?
>
Probably not.  I wrote a small test program using a list several levels 
deep, each list containing 5 sublists at each level and finally just a 
list of numbers.  Flattening 1000 times took about 3.9 seconds for the 
one creating a list at each level, and 3.2 for the one not creating the 
list at each level.

Brian Vanderburg II