全面剖析Python字典dict的各方法及各操作的内部实现

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构造方法

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class dict(object):
    """
    dict() -> new empty dictionary
    dict(mapping) -> new dictionary initialized from a mapping object's
        (key, value) pairs
    dict(iterable) -> new dictionary initialized as if via:
        d = {}
        for k, v in iterable:
            d[k] = v
    dict(**kwargs) -> new dictionary initialized with the name=value pairs
        in the keyword argument list.  For example:  dict(one=1, two=2)
    """

除了字面量的语法{}构造字典，也可以用dict()构造字典。参数有几种，示例如下：

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>>> D = {}
>>> D
{}
>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D
{'A': 1, 'B': 2, 'C': 3}
>>> D = dict()
>>> D
{}
>>> D = dict({'A': 1, 'B': 2, 'C': 3})
>>> D
{'A': 1, 'B': 2, 'C': 3}
>>> D = dict([('A', 1), ('B', 2), ('C', 3)])
>>> D
{'A': 1, 'B': 2, 'C': 3}
>>> D = dict(A=1, B=2, C=3)
>>> D
{'C': 3, 'B': 2, 'A': 1}
>>>

清空字典(删除字典中所有元素)

方法：

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def clear(self):
    """ D.clear() -> None.  Remove all items from D. """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D.clear()
>>> D
{}
>>>

字典浅复制

方法：

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def copy(self):
    """ D.copy() -> a shallow copy of D """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> d = D.copy()
>>> d
{'A': 1, 'B': 2, 'C': 3}
>>>

由键组成的可迭代对象创建新字典

方法：

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@staticmethod
def fromkeys(*args, **kwargs): # real signature unknown
    """ Returns a new dict with keys from iterable and values equal to value. """
    pass

示例：

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>>> dict.fromkeys({'name', 'age', 'sex'})
{'age': None, 'sex': None, 'name': None}
>>> dict.fromkeys({'name', 'age', 'sex'}, 10)
{'age': 10, 'sex': 10, 'name': 10}
>>>

返回指定键的值，如果键不存在返回默认值

方法：

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def get(self, k, d=None):
    """ D.get(k[,d]) -> D[k] if k in D, else d.  d defaults to None. """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D.get('A')
1
>>> D.get('D', 0)
0
>>>

得到字典中所有元素的可迭代对象

方法：

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def items(self):
    """ D.items() -> a set-like object providing a view on D's items """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D.items()
dict_items([('A', 1), ('B', 2), ('C', 3)])
>>> for k, v in D.items():
...     print(k, v)
... 
A 1
B 2
C 3
>>> list(D.items())
[('A', 1), ('B', 2), ('C', 3)]
>>>

得到字典中所有键的可迭代对象

方法：

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def keys(self):
    """ D.keys() -> a set-like object providing a view on D's keys """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D.keys()
dict_keys(['A', 'B', 'C'])
>>> for k in D.keys():
...     print(k)
... 
A
B
C
>>> list(D.keys())
['A', 'B', 'C']
>>>

删除指定键，并返回对应的值

方法：

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def pop(self, k, d=None):
    """
    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
    """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D.pop('A')
1
>>> D
{'B': 2, 'C': 3}
>>> D.pop('D', 0)
0
>>> D
{'B': 2, 'C': 3}
>>>

删除并返回键值对所组成的元组

方法：

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def popitem(self):
    D.popitem() -> (k, v), remove and return some (key, value) pair as a
    2-tuple; but raise KeyError if D is empty.
    """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D.popitem()
('A', 1)
>>> D.popitem()
('B', 2)
>>> D.popitem()
('C', 3)
>>> D
{}
>>>

键存在则返回对应的值，不存在则设置值并返回值

方法：

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def setdefault(self, k, d=None):
    """ D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D.setdefault('A', 0)
1
>>> D
{'A': 1, 'B': 2, 'C': 3}
>>> D.setdefault('D', 0)
0
>>> D
{'D': 0, 'A': 1, 'B': 2, 'C': 3}
>>>

用另一个可迭代对象更新字典

方法：

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def update(self, E=None, **F):
    """
    D.update([E, ]**F) -> None.  Update D from dict/iterable E and F.
    If E is present and has a .keys() method, then does:  for k in E: D[k] = E[k]
    If E is present and lacks a .keys() method, then does:  for k, v in E: D[k] = v
    In either case, this is followed by: for k in F:  D[k] = F[k]
    """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D.update({'A': 0, 'D': 1})
>>> D
{'D': 1, 'A': 0, 'B': 2, 'C': 3}
>>> D.update({'A': 0, 'D': 1}, A=1, D=4)
>>> D
{'D': 4, 'A': 1, 'B': 2, 'C': 3}
>>>

得到字典中所有值的可迭代对象

方法：

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def values(self):
    """ D.values() -> an object providing a view on D's values """
    pass

示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D.values()
dict_values([1, 2, 3])
>>> for v in D.values():
...     print(v)
... 
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2
3
>>> list(D.values())
[1, 2, 3]
>>>

字典对外接口的内部实现

in语句检测字典中是否有某键

内部实现方法：

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def __contains__(self, *args, **kwargs):
    """ True if D has a key k, else False. """
    pass

对外接口示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> 'A' in D
True
>>> 'D' in D
False
>>>

len()函数返回字典长度

内部实现方法：

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def __len__(self, *args, **kwargs):
    """ Return len(self). """
    pass

对外接口示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> len(D)
3
>>>

repr()返回字典对象字符串表示形式

内部实现方法：

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def __repr__(self, *args, **kwargs):
    """ Return repr(self). """
    pass

对外接口示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> repr(D)
"{'A': 1, 'B': 2, 'C': 3}"
>>>

dict()构造字典

内部实现方法：

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def __init__(self, seq=None, **kwargs):
    """
    dict() -> new empty dictionary
    dict(mapping) -> new dictionary initialized from a mapping object's
        (key, value) pairs
    dict(iterable) -> new dictionary initialized as if via:
        d = {}
        for k, v in iterable:
            d[k] = v
    dict(**kwargs) -> new dictionary initialized with the name=value pairs
        in the keyword argument list.  For example:  dict(one=1, two=2)
    # (copied from class doc)
    """
    pass
 
@staticmethod
def __new__(*args, **kwargs): # real signature unknown
    """ Create and return a new object.  See help(type) for accurate signature. """
    pass

对外接口示例：

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>>> D = dict()
>>> D
{}
>>> D = dict({'A': 1, 'B': 2, 'C': 3})
>>> D
{'A': 1, 'B': 2, 'C': 3}
>>> D = dict([('A', 1), ('B', 2), ('C', 3)])
>>> D
{'A': 1, 'B': 2, 'C': 3}
>>> D = dict(A=1, B=2, C=3)
>>> D
{'C': 3, 'B': 2, 'A': 1}
>>>

通过比较运算符比较字典

内部实现方法：

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def __eq__(self, *args, **kwargs):
    """ Return self==value. """
    pass
def __ne__(self, *args, **kwargs):
    """ Return self!=value. """
    pass
def __ge__(self, *args, **kwargs):
    """ Return self>=value. """
    pass
def __gt__(self, *args, **kwargs):
    """ Return self>value. """
    pass
def __le__(self, *args, **kwargs):
    """ Return self<=value. """
    pass
def __lt__(self, *args, **kwargs):
    """ Return self<value. """
    pass

对外接口示例：

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>>> D1 = {'A': 1, 'B': 2, 'C': 3}
>>> D2 = {'B': 2, 'C': 3, 'A': 1}
>>> D1 == D2
True
>>>

通过键的方式访问、设置、删除元素

内部实现方法：

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def __getitem__(self, y):
    """ x.__getitem__(y) <==> x[y] """
    pass
def __setitem__(self, *args, **kwargs):
    """ Set self[key] to value. """
    pass
def __delitem__(self, *args, **kwargs):
    """ Delete self[key]. """
    pass

对外接口示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> D['A']
1
>>> D['B'] = 0
>>> D
{'A': 1, 'B': 0, 'C': 3}
>>> del D['C']
>>> D
{'A': 1, 'B': 0}
>>>

getattr()得到字典某属性

内部实现方法：

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def __getattribute__(self, *args, **kwargs):
    """ Return getattr(self, name). """
    pass

对外接口示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> dir(D)
['__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values']
>>> getattr(D, '__class__')
<class 'dict'>
>>> getattr(D, 'values')
<built-in method values of dict object at 0x7fc3e354da88>
>>>

创建可迭代对象

内部实现方法：

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def __iter__(self, *args, **kwargs):
    """ Implement iter(self). """
    pass

对外接口示例：

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>>> D = {'A': 1, 'B': 2, 'C': 3}
>>> iter(D)
<dict_keyiterator object at 0x7f0f1f0f9e08>
>>>

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