Iterating through a nested list

[Steven D'Aprano]

On Sat, 15 Apr 2006 22:36:17 -0500, CamelR wrote:

> I have a newbie question, and I have seen reference to this mentioned
> in many places, but all just say "this has been discussed frequently
> in (other places) so we won't discuss it here..." and I am unable to
> find the actual answer...
> 
> I expected :
> 
> Array = ["topdir", ["/subdir1", ["/file1", "/file2"], "/subdir2",
> ["/file1", "/file2"], "/subdir3", ["/file1", "/file2"]]]
> 
> for i in range(len(Array)):
>   print Array[i]
>   for x in range(len(Array[i])):
>     print Array[i][x]
>     for y in range(len(Array[i][x])):
>       print Array[i][x][y]

If you don't actually need the index of each item, don't use for i in
range(), just iterate over the list:

for obj in Array:
    print obj

Notice that your list has two items, the first is a string and the second
is a sub-list. The natural way of proceeding would be:

for obj in Array:
    for item in obj:
        print item

but that gives too much: it iterates over the string, giving you
individual characters.

Instead, do this:

for obj in Array:
    if isinstance(obj, list):
        for item in obj:
            print item
    else:
        print obj


which helps, but not enough: it only prints two levels down, and your data
has three. This time -- next time it might have two, or fifteen, or one...

Instead, create a helper function like this:

def walk(seq):
    """Walk over a sequence of items, printing each one in turn, and
    recursively walking over sub-sequences.
    """
    print seq
    if isinstance(seq, list):
        for item in seq:
            walk(item)


Notice that this function calls itself.

This version of walk() prints the entire sequence at every level. Here is
a version which doesn't:

def walk2(seq):
    if isinstance(seq, list):
        for item in seq:
            walk2(item)
    else:
        print seq


-- 
Steven.