Question

I need to be able to take lists containing strings and nested lists of strings like the following:

['parent', ['child', 'child2', ['grandchild', ['ggrandchild'], 'grandchild2'], 'child3'], '2parent', '3parent' ['3child', ['3grandchild']]]

And print strings for each parent, each parent-child, each parent-child-grandchild, etc.:

'parent'
'parent_child'
'parent_child2'
'parent_child2_grandchild'
'parent_child2_grandchild_ggrandchild'
'parent_child2_grandchild2'
'parent_child3"
'2parent'
...
etc

I have been able to get it to work to a depth of two nested levels with the following code:

def list_check(parsed_list):
    for item in parsed_list:

        if type(item) != list and prefix == []: 
            prefix.append(str(item))
            print item

        elif type(item) != list and prefix != []: 
            print prefix[0]+"-"+item

        elif type(item) == list:
            list_check(item)

        else:
            pass

But I’m struggling with getting it to work for arbitrary nesting depth. I’ve taken a basic approach of tracking the nesting depth through a stack, but my implementation is broken in an obvious way that I don’t know how to fix.

It currently deletes what is in the stack even if there are still other children following. What I want it to do is only pop items out of the relevant stacks if the sublist is ending.

prefix = []
nesting_depth = []

def list_check(parsed_list):
    for item in parsed_list:

        if type(item) == list:

            nesting_depth.append('1')
            list_check(item)

        elif type(item) != list and prefix == []: 
            prefix.append(str(item))
            print item

        elif type(item) != list and prefix != []: #this where i need another condition like 'and item is last in current sublist'
            print prefix[0:len(nesting_depth)]+"-"+item
            prefix.pop()

        else:
            pass

How do I reference something like ‘if it’s the last item in the current sublist of parsed_list’ in a way that accommodates the recursion of the overall function?

Answer 1

A dictionary would be much better suited to your needs. But here’s what you can do with the data structure you have:

def format_tree(tree):                                                                                            
    prefix = ''
    for node in tree:
        if isinstance(node, basestring):
            prefix = node
            yield node
        else:
            for elt in format_tree(node):
                yield prefix + '_' + elt 

A simple test:

>>> a = ['parent', ['child', 'child2', ['grandchild', ['ggrandchild']], 'child3']]
>>> print '\n'.join(format_tree(a))
parent
parent_child
parent_child2
parent_child2_grandchild
parent_child2_grandchild_ggrandchild
parent_child3