#defaultdict will not always generate default value
#if you didn't assign default data type to it, using defaultdict[key] to find the key might get an error

#defaultdict has default data type
>>> x = defaultdict(int)
>>> x
defaultdict(<type 'int'>, {})
>>> x["abc"]
0
>>> x
defaultdict(<type 'int'>, {'abc': 0})


#defaultdict has no default data type
>>> x = defaultdict()
>>> x
defaultdict(None, {})
>>> x["abc"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'abc'

#for safety, instead, we can use get() to find the key
>>> x.get("abc") #will return nothing, but will not cause error
>>>