# [Chicago] Resolving lists within lists within lists within .....

Aaron Elmquist elmq0022 at umn.edu
Fri Feb 19 07:59:36 EST 2016

```Here's one last approach that is stack based.  There is some clean up to do
here for sure (I'm mutating the original list for one), but the point is to
illustrate an approach that is not recursive.

def flatten_big_list(lst):
stack = []
while(lst):
top = lst.pop(0)
while(isinstance(top,list)):
temp = top.pop(0)
if top:
lst.insert(0,top)
top = temp
stack.append(top)
return stack

def flatten_big_list_gen(lst):
while(lst):
top = lst.pop(0)
while(isinstance(top,list)):
temp = top.pop(0)
if top:
lst.insert(0,top)
top = temp
yield top

print(flatten_big_list([1, [2, [3, [4, 5]]]]))
print(list(flatten_big_list_gen([1, [2, [3, [4, 5]]]])))

Feedback is always welcome.

On Thu, Feb 18, 2016 at 9:29 PM, Mark Graves <mgraves87 at gmail.com> wrote:

> Doug,
>
>
> "The" answer to why is recursion expensive vs iteration is stack traces.
> try it yourself as mentioned here
> <http://t.yesware.com/tt/6640a48a14dbdef70b47105ac6b72156559fc5a6/5ba2375237a9fdc8efa681b19014981f/dda1509570b2b5d9d162e6293a1b3f07/stackoverflow.com/questions/22893139/why-is-a-function-method-call-in-python-expensive>
> .
>
> Recursion means creating more functions / stack traces.
>
> wrote:
>
>> Phil,
>>
>> That's generally true, but one small correction. Aaron's solution won't
>> actually won't flatten strings, as they don't have "__iter__" methods. They
>> implement iteration because they take sequential numeric indexes starting
>> at 0, and raise an IndexError after the index passed is too large.
>>
>> On Feb 18, 2016 19:22, "Robare, Phillip (TEKSystems)" <proba at allstate.com>
>> wrote:
>>
>>> Aaron, unlike Massimo’s elegant one-liner you don’t check that what you
>>> are iterating over is a list.  Since Python will happily iterate over
>>> strings, dictionaries, and much more you quickly get into problems when the
>>> list includes more types than lists and numbers.  I recount this from
>>> experience when I tried to throw together a flatten routine and pass it a
>>>
>>>
>>>
>>> Phil Robare
>>>
>>>
>>>
>>> *<snip/>*
>>>
>>>
>>>
>>> On Thu, Feb 18, 2016 at 1:43 PM, Aaron Elmquist <elmq0022 at umn.edu>
>>> wrote:
>>>
>>> Douglas,
>>>
>>> Here's one more version for you and the rest of the list. It's based on
>>> Brad's code.  I will let you think about why this version might be better
>>> or worse.  Also, recursion is great.  It's just too bad it's not one of
>>> python's strong points.
>>>
>>>
>>> def flatten(lst):
>>>     for item1 in lst:
>>>         if hasattr(item1, '__iter__'):
>>>             for item2 in flatten(item1):
>>>                 yield item2
>>>         else:
>>>             yield item1
>>>
>>> print([x for x in flatten([1, [2,3,[4,5,6,[7,8,9]]]]) if x%2 == 1])
>>>
>>> y = flatten([1, [2,3,[4,5,6,[7,8,9]]]])
>>>
>>> print(next(y))
>>> print(next(y))
>>> print(next(y))
>>> .
>>> .
>>> .
>>> <snip/>
>>>
>>> On Wed, Feb 17, 2016 at 9:48 PM, DiPierro, Massimo <
>>> MDiPierro at cs.depaul.edu> wrote:
>>>
>>> here is a one liner:
>>>
>>> def flatten(x):
>>>     return [z for y in x for z in flatten(y)] if isinstance(x,list) else
>>> [x]
>>>
>>>
>>>
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>>> https://mail.python.org/mailman/listinfo/chicago
>>>
>>>
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>>
>
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