Python function saving results to same dict

Question

I`m trying to solve an exercise by using python and i think i solved it. When i run single test with my function it works well. But when its multiple checks i got errors. Every function saving results to the same dictionary. I tried to read about visibility of variables but i got nothing. Please give me a link where can i read about this topic.

def flatten(dicts, a='', free={}):
    for k, v in dicts.iteritems():
        if a in dicts.keys():
                    a = ""
        if a != "":
            b = "/"
        else:
            b = ""
        if isinstance(v, dict):
            if len(v.keys()) == 0:
                free[a+b+k] = ""
            else:
                a += b+k
                flatten(v, a)
        else:
            free[a+b+k] = v
    return free


print flatten({"key": "value"}) == {"key": "value"}
print flatten({"key": {"deeper": {"more": {"enough": "value"}}}}) == {"key/deeper/more/enough": "value"}
print flatten({"name": {
                        "first": "One",
                        "last": "Drone"},
                    "job": "scout",
                    "recent": {},
                    "additional": {
                        "place": {
                            "zone": "1",
                            "cell": "2"}}}
    ) == {"name/first": "One",
          "name/last": "Drone",
          "job": "scout",
          "recent": "",
          "additional/place/zone": "1",
          "additional/place/cell": "2"}

I got this in the end:

{'key/deeper/more/enough': 'value', 'additional/place/zone': '1',
'job': 'scout', 'additional/place/cell': '2', 'name/first': 'One', 
'name/last': 'Drone', 'key': 'value', 'recent': ''}