defaultdictを返すdefaultdictを作って、KeyErrrorの発生しない世界を作る（+JSONのパースでの利用例） ref: http://qiita.com/shrkw/items/6f872ece6b29e160c0ec

file0.txt

>>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
>>> d = defaultdict(list)
>>> for k, v in s:
...     d[k].append(v)
...
>>> d.items()
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

file1.txt

def _factory():
    return collections.defaultdict(_factory)

file2.txt

>>> import collections
>>> def _factory():
...     return collections.defaultdict(_factory)
... 
>>> d = collections.defaultdict(_factory)
>>> d['a']
defaultdict(<function _factory at 0x105e93aa0>, {})
>>> d['a']['b']
defaultdict(<function _factory at 0x105e93aa0>, {})
>>> d['a']['b'][1]
defaultdict(<function _factory at 0x105e93aa0>, {})
>>> d[1][1][0][1][1][1][1][1]
defaultdict(<function _factory at 0x105e93aa0>, {})

file3.txt

{  
  "class":{  
    "id":1,
    "subject":"Math",
    "students":[  
      {  
        "name":"Alice",
        "age":30
      },
      {  
        "name":"Bob",
        "age":40,
        "address":{  
          "country":"JP"
        }
      },
      {  
        "name":"Charlie",
        "age":20,
        "address":{  
          "country":"US",
          "state":"MA",
          "locality":"Boston"
        }
      }
    ]
  }
}

file4.txt

In [47]: j = json.loads(s)

In [54]: for student in j["class"]["students"]:
    print(student["name"])
   ....:     
Alice
Bob
Charlie

file5.txt

In [55]: for student in j["class"]["students"]:
    print(student["address"]["state"])
   ....:
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-55-69836c86e040> in <module>()
      1 for student in j["class"]["students"]:
----> 2     print(student["address"]["state"])
      3 

KeyError: 'address'

file6.txt

def _hook(d):
    return collections.defaultdict(_factory, d)

file7.txt

In [75]: j2 = json.loads(s, object_hook=_hook)
In [83]: for student in j2["class"]["students"]:
   ....:     print(student["address"]["state"])
   ....:     
defaultdict(<function _factory at 0x10a57ccf8>, {})
defaultdict(<function _factory at 0x10a57ccf8>, {})
MA

file8.txt

In [91]: def _dd(v, alt_val="default_state"):
    return alt_val if isinstance(v, collections.defaultdict) and len(v) == 0 else v
   ....: 

In [92]: for student in j2["class"]["students"]:
    print(_dd(student["address"]["state"]))
   ....:     
default_state
default_state
MA