Recursive JSON walk in Python [closed]

I wrote this JSON walker to help with moving objects around while updating Legacy JSONs to match a new schema. Please let me know how I could further optimize it, or more importantly, make it easier to understand and maintain.

def walk(self, obj=None):
    """Recursively walks JSON and returns a list of all 
    key combinations. 

    sample input JSON:
    { 
    'key1' : 'value1',
        'key2':'value2',
        'key3':{'key3a':'value3a'},
        'key4':{'key4a': [
                            { 
                                'key4aa':'value4aa'
                            }
                        ],
                        [
                            { 
                                'key4ab':'value4ab',
                                'key4ac':'value4ac'
                            }  
                        ]
            'key4b':'value4b'
        }
    }
    corresponding output:
        [
            ['key1' : 'value1'],
            ['key1', 'key2' : 'value2'],
            ['key1', 'key3', 'key3a'],
            ['key1', 'key4', key4a, 0, 'key4aa'],
            ['key1', 'key4', key4a, 1, 'key4ab'],
            ['key1', 'key4', key4a, 1, 'key4ac'],
            ['key1', 'key4', 'key4b']
        ]
    """
    # python default values are calculated when the function
    # is defined, not when it is called. This is a work-around
    if obj is None:
        obj = self._json            

    # optimization: these are the JSON schema default values.
    #   they can not have child elements
    if obj == "" or obj == -1:
        return []
    # walk a dictionary
    elif isinstance(obj, Mapping):
        flat_tree = []
        for key, val in obj.iteritems():
            # make a new branch for each child key
            children = self.walk(val)
            if children == []:
                flat_tree.append([key])
            else:
                flat_tree.extend([[key] + child for child in children])
        return flat_tree
    # walk lists
    elif isinstance(obj, Iterable) and not isinstance(obj, StringTypes):
        flat_tree = []
        for ii, val in enumerate(obj):
            # make a new branch for each child key
            children = self.walk(val)
            if children == []:
                flat_tree.append([ii])
            else:
                flat_tree.extend([[ii] + child for child in children])
        return flat_tree
    # All possible iterable datatypes have been ruled out
    #   so this element is a JSON value type: (bool, int, str, float, ect.)
    else:
        return []