Find all paths through a tree (nested dicts) from top to bottom

Question

EDIT: See below for a suggested answer and how it's not quite right yet.

There are many similar questions to this one on Stack Overflow, but none exactly like it in Python. I'm a programming novice, so please go easy.

I have a tree of nested dictionaries, like this:

[{'word': 'The',
  'next': [{'word': 'End',
            'next': None},
           {'word': 'quick',
            'next': [{'word': 'brown',
                      'next': [{'word': 'fox',
                                'next': None}]}]},
           {'word': 'best',
            'next': [{'word': 'of',
                      'next': [{'word': 'times',
                                'next': None}]}]}]}] 

I want to flatten all paths from top to bottom and end up with this:

[[{'word': 'The'},
  {'word': 'End'}],

 [{'word': 'The'},
  {'word': 'quick'},
  {'word': 'brown'},
  {'word': 'fox'}],

 [{'word': 'The'},
  {'word': 'best'},
  {'word': 'of'},
  {'word': 'times'}]]

I made a lovely little recursive function that created the original structure in the first place, but I'm having a hard time unrecursivizing it. This is as far as I got:

def flatten_combinations(result_tree, current_combo = None, all_combos = None):
    if current_combo is None:
        current_combo = []
    if all_combos is None:
        all_combos = []
    if result_tree is None:
        all_combos.append(current_combo)
        return
    for word in result_tree:
        current_combo.append({'word': word['word']})
        flatten_combinations(word['next'], current_combo, all_combos)
    return current_combo

…which returns this:

[{'word': 'The'},
 {'word': 'End'},
 {'word': 'quick'},
 {'word': 'brown'},
 {'word': 'fox'},
 {'word': 'best'},
 {'word': 'of'},
 {'word': 'times'}]

…which is clearly somewhat close, but not quite right.

I know that function is probably horribly un-Pythonic, but I'm teaching myself programming, so I'm not even trying to take advantage of possibly-existent language features that would let me elide over thinking through this stuff from scratch (” he said, posting to a Q&A site in the hope its members would help him elide a bit of thought).

So: what am I doing wrong?

EDIT: Moshe below corrected a couple of problems:

def flatten_combinations(result_tree, current_combo = None, all_combos = None):
    if current_combo is None:
        current_combo = []
    if all_combos is None:
        all_combos = []
    if result_tree is None:
        all_combos.append(current_combo)
        return
    for word in result_tree:
        current_combo = current_combo[:]
        current_combo.append({'word': word['word']})
        flatten_combinations(word['next'], current_combo, all_combos)
    return all_combos 

This is closer yet, but not quite right:

[{'word': 'The'}, 
 {'word': 'End'}],

[{'word': 'The'},
 {'word': 'End'},
 {'word': 'quick'},
 {'word': 'brown'},
 {'word': 'fox'}],

[{'word': 'The'},
 {'word': 'End'},
 {'word': 'quick'},
 {'word': 'best'},
 {'word': 'of'},
 {'word': 'times'}]]

Answer 1

There are two minor mistakes in that:

1) You return current_combo instead of all_combos. That only gives you your last result

2) On each iteration, you modify current_combo. Make a copy first!

new_current_combo = current_combo[:]
new_current_combo.append({'word': word['word']})
flatten_combinations(word['next'], new_current_combo, all_combos)

Full code:

def flatten_combinations(result_tree, current_combo=None, all_combos=None):
    if current_combo is None:
        current_combo = []
    if all_combos is None:
        all_combos = []
    if result_tree is None:
        all_combos.append(current_combo)
        return
    for word in result_tree:
        new_current_combo = current_combo[:]
        new_current_combo.append({'word': word['word']})
        flatten_combinations(word['next'], new_current_combo, all_combos)
    return all_combos 

Answer 2

If you pass in a copy of current_combo on the recursive call then you won't lose your current path on the next iteration of your for loop.

Also, you don't need to return current_combo since the result isn't used in recursion. You might want to return all_combos instead and not take it as a parameter. Alternatively, you could have a recursive function and none recursive function, with the no-recursive function creating the list for all_combos and passing it into the recursive function so that the recursive function could assume all_combos was set.