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.