Flattening lists

[mk]

Brian Allen Vanderburg II wrote:
>> def flatten(x):
>>     res = []
>>     for el in x:
>>         if isinstance(el,list):
>>             res.extend(flatten(el))
>>         else:
>>             res.append(el)
>>     return res

> 
> I think it may be just a 'little' more efficient to do this:
> 
> def flatten(x, res=None):
>    if res is None:
>       res = []
> 
>    for el in x:
>       if isinstance(el, (tuple, list)):
>          flatten(el, res)
>       else:
>          res.append(el)
> 
>    return res


Hmm why should it be more efficient? extend operation should not be very 
costly?

Regards,
mk