Flattening lists

[Sion Arrowsmith]

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

]? extend operation should not be very costly?

It's not a question of extend/append, it's the fact that your
original function creates (and destroys) a new list for every
recursive call. Which, if you've got large nested lists, will
have an impact.

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