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Re: Understanding how is a function evaluated using recursion

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On 26/09/2013 00:24, Arturo B wrote:
> Hi, I'm doing Python exercises and I need to write a function to flat nested lists
> as this one:
>
> [[1,2,3],4,5,[6,[7,8]]]
>
> To the result:
>
> [1,2,3,4,5,6,7,8]
>
> So I searched for example code and I found this one that uses recursion (that I don't understand):
>
> def flatten(l):
>      ret = []
>      for i in l:
>          if isinstance(i, list) or isinstance(i, tuple):
>              ret.extend(flatten(i)) #How is flatten(i) evaluated?
>          else:
>              ret.append(i)
>      return ret
>
> So I know what recursion is, but I don't know how is
>
>                         flatten(i)
>
> evaluated, what value does it returns?
>
Try a simpler version first:

def flatten(l):
     ret = []
     for i in l:
         if isinstance(i, list) or isinstance(i, tuple):
             # Append the contents of the item.
             ret.extend(i)
         else:
             # Append the item itself.
             ret.append(i)
     return ret

In this example, flatten([[1,2,3],4,5,[6,[7,8]]]) returns [1,2,3,4,5,6,
[7,8]].

The problem here is that a sublist can itself contain a list.

It would be nice if there were a function which, when given [6,[7,8]],
would return [6,7,8] so that you could append those items.

But that's exactly what flatten does!

Try adding prints to tell you what was passed in and what is returned.

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Thread

Understanding how is a function evaluated using recursion Arturo B <a7xrturodev@gmail.com> - 2013-09-25 16:24 -0700
  Re: Understanding how is a function evaluated using recursion Josh English <Joshua.R.English@gmail.com> - 2013-09-25 16:59 -0700
  Re: Understanding how is a function evaluated using recursion MRAB <python@mrabarnett.plus.com> - 2013-09-26 01:07 +0100
  Re: Understanding how is a function evaluated using recursion Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-09-26 00:10 +0000
  Re: Understanding how is a function evaluated using recursion Dave Angel <davea@davea.name> - 2013-09-26 00:26 +0000
  Re: Understanding how is a function evaluated using recursion Terry Reedy <tjreedy@udel.edu> - 2013-09-25 21:12 -0400
  Re: Understanding how is a function evaluated using recursion rusi <rustompmody@gmail.com> - 2013-09-25 21:04 -0700
  Re: Understanding how is a function evaluated using recursion Neil Cerutti <neilc@norwich.edu> - 2013-09-26 14:18 +0000
    Re: Understanding how is a function evaluated using recursion Neil Cerutti <neilc@norwich.edu> - 2013-09-26 14:23 +0000
      Re: Understanding how is a function evaluated using recursion Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-09-28 13:24 -0700
  Re: Understanding how is a function evaluated using recursion rusi <rustompmody@gmail.com> - 2013-09-29 20:41 -0700

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