>>> from collections import defaultdict

>>> s='mmississippi'

>>> d=defaultdict(int)

>>> for i in s:

    d[i]+=1

>>> print dict(d)

{'i': 4, 'p': 2, 's': 4, 'm': 2}

>>> from collections import defaultdict

>>> def ini():

    return 3

>>> d=defaultdict(ini)

>>> d[1]   #可见,不一定要是list,int,float,dict这些工厂函数,自定义函数也能成功.

3

>>> d

defaultdict(<function ini at 0x020CA5F0>, {1: 3})

>>> d[2]

3

>>> d

defaultdict(<function ini at 0x020CA5F0>, {1: 3, 2: 3})

>>> 

>>> d=defaultdict(ini,{1:2})

>>> d

defaultdict(<function ini at 0x020CA5F0>, {1: 2})

>>> d[3]

3

>>> d

defaultdict(<function ini at 0x020CA5F0>, {1: 2, 3: 3})

>>> d=defaultdict(ini,a=1,b=2)

>>> d

defaultdict(<function ini at 0x020CA5F0>, {'a': 1, 'b': 2})

>>> d=defaultdict(ini,[(1,3)])

>>> d

defaultdict(<function ini at 0x020CA5F0>, {1: 3})

    class defaultdict(__builtin__.dict)

     |  defaultdict(default_factory) --> dict with default factory

     |

     |  The default factory is called without arguments to produce

     |  a new value when a key is not present, in __getitem__ only.

     |  A defaultdict compares equal to a dict with the same items.

     |

     |  Method resolution order:

     |      defaultdict

     |      __builtin__.dict

     |      __builtin__.object

     |

     |  Methods defined here:

     |

     |  __copy__(...)

     |      D.copy() -> a shallow copy of D.

     |

     |  __getattribute__(...)

     |      x.__getattribute__('name') <==> x.name

     |

     |  __init__(...)

     |      x.__init__(...) initializes x; see help(type(x)) for signature

     |

     |  __missing__(...)

     |      __missing__(key) # Called by __getitem__ for missing key; pseudo-code:

     |      if self.default_factory is None: raise KeyError((key,))

     |      self[key] = value = self.default_factory()

     |      return value

     |

     |  __reduce__(...)

     |      Return state information for pickling.

     |

     |  __repr__(...)

     |      x.__repr__() <==> repr(x)

     |

     |  copy(...)

     |      D.copy() -> a shallow copy of D.

     |

     |  ----------------------------------------------------------------------

     |  Data descriptors defined here:

     |

     |  default_factory

     |      Factory for default value called by __missing__().

     |

     |  ----------------------------------------------------------------------

     |  Methods inherited from __builtin__.dict:

     |

     |  __cmp__(...)

     |      x.__cmp__(y) <==> cmp(x,y)

     |

     |  __contains__(...)

     |      D.__contains__(k) -> True if D has a key k, else False

     |

     |  __delitem__(...)

     |      x.__delitem__(y) <==> del x[y]

     |

     |  __eq__(...)

     |      x.__eq__(y) <==> x==y

     |

     |  __ge__(...)

     |      x.__ge__(y) <==> x>=y

     |

     |  __getitem__(...)

     |      x.__getitem__(y) <==> x[y]

     |

     |  __gt__(...)

     |      x.__gt__(y) <==> x>y

     |

     |  __iter__(...)

     |      x.__iter__() <==> iter(x)

     |

     |  __le__(...)

     |      x.__le__(y) <==> x<=y

     |

     |  __len__(...)

     |      x.__len__() <==> len(x)

     |

     |  __lt__(...)

     |      x.__lt__(y) <==> x<y

     |

     |  __ne__(...)

     |      x.__ne__(y) <==> x!=y

     |

     |  __setitem__(...)

     |      x.__setitem__(i, y) <==> x[i]=y

     |

     |  __sizeof__(...)

     |      D.__sizeof__() -> size of D in memory, in bytes

     |

     |  clear(...)

     |      D.clear() -> None.  Remove all items from D.

     |

     |  fromkeys(...)

     |      dict.fromkeys(S[,v]) -> New dict with keys from S and values equal to v.

     |      v defaults to None.

     |

     |  get(...)

     |      D.get(k[,d]) -> D[k] if k in D, else d.  d defaults to None.

     |

     |  has_key(...)

     |      D.has_key(k) -> True if D has a key k, else False

     |

     |  items(...)

     |      D.items() -> list of D's (key, value) pairs, as 2-tuples

     |

     |  iteritems(...)

     |      D.iteritems() -> an iterator over the (key, value) items of D

     |

     |  iterkeys(...)

     |      D.iterkeys() -> an iterator over the keys of D

     |

     |  itervalues(...)

     |      D.itervalues() -> an iterator over the values of D

     |

     |  keys(...)

     |      D.keys() -> list of D's keys

     |

     |  pop(...)

     |      D.pop(k[,d]) -> v, remove specified key and return the corresponding value.

     |      If key is not found, d is returned if given, otherwise KeyError is raised

     |

     |  popitem(...)

     |      D.popitem() -> (k, v), remove and return some (key, value) pair as a

     |      2-tuple; but raise KeyError if D is empty.

     |

     |  setdefault(...)

     |      D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D

     |

     |  update(...)

     |      D.update([E, ]**F) -> None.  Update D from dict/iterable E and F.

     |      If E present and has a .keys() method, does:     for k in E: D[k] = E[k]

     |      If E present and lacks .keys() method, does:     for (k, v) in E: D[k] = v

     |      In either case, this is followed by: for k in F: D[k] = F[k]

     |

     |  values(...)

     |      D.values() -> list of D's values

     |

     |  viewitems(...)

     |      D.viewitems() -> a set-like object providing a view on D's items

     |

     |  viewkeys(...)

     |      D.viewkeys() -> a set-like object providing a view on D's keys

     |

     |  viewvalues(...)

     |      D.viewvalues() -> an object providing a view on D's values

     |

     |  ----------------------------------------------------------------------

     |  Data and other attributes inherited from __builtin__.dict:

     |

     |  __hash__ = None

     |

     |  __new__ = <built-in method __new__ of type object>

     |      T.__new__(S, ...) -> a new object with type S, a subtype of T