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Date: Thu, 26 Sep 2013 01:07:01 +0100
From: MRAB <python@mrabarnett.plus.com>
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Subject: 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.