Python中的魔法属性

# __doc__
class Foo:
    """ 描述类信息，这是用于测试的类 """
    
    def func(self):
        pass

    
# ipython 测验
In [2]: Foo.__doc__
Out[2]: ' 描述类信息，这是用于测试的类 '
    

# __module__、__class__
# oop.py
class Student(object):
    
    def __init__(self, name):
        self.name = name
        
        
# main.py
from oop import Student

s = Student()
print(s.__module__)  # 输出 oop 即：输出模块
print(s.__class__)   # 输出 <class 'oop.Student'> 即：输出类

# __init__ 、 __new__
class Student(object):

    def __init__(self, name, age):
        print('__init__() called')
        self.name = name
        self.age = age

    def __new__(cls, *args, **kwargs):
        print('__new__() called')
        print(cls, args, kwargs)
        return super().__new__(cls)
  

# ipython 测验
In [26]: s1 = Student('hui', age=21)
__new__() called
<class '__main__.Student'> ('hui',) {'age': 21}
__init__() called

In [27]: s2 = Student('jack', age=20)
__new__() called
<class '__main__.Student'> ('jack',) {'age': 20}
__init__() called

# __del__
class Foo:
    def __del__(self):
        print('__del__() called')

        
# ipython 测验
In [29]: f = Foo()

In [30]: del f
__del__() called

# __call__
class Rect(object)
    """
    调用实例对象来改变矩形的位置
    """

    def __init__(self, x, y):
        # x, y代表矩形坐标
        self.x, self.y = x, y

    def __call__(self, x, y):        
        # 改变实体的位置
        self.x, self.y = x, y


# ipython 测验
In [33]: r = Rect(10, 10)

In [34]: r.x, r.y
Out[34]: (10, 10)

In [35]: r(0, 0)

In [36]: r.x, r.y
Out[36]: (0, 0)

In [37]: r(100, 100)

In [38]: r.x, r.y
Out[38]: (100, 100)

# __dict__
class Student(object):

    def __init__(self, name, age):
        self.name = name
        self._age = age

    @property
    def age(self):
        return self._age

    
# ipython 测验
In [47]: # 获取类属性

In [48]: Student.__dict__
Out[48]:
mappingproxy({'__module__': '__main__',
              '__init__': <function __main__.Student.__init__(self, name, age)>,
              'age': <property at 0x210e2a005e8>,
              '__dict__': <attribute '__dict__' of 'Student' objects>,
              '__weakref__': <attribute '__weakref__' of 'Student' objects>,
              '__doc__': None})

In [49]: # 获取实例对象的属性

In [50]: s = Student('hui', 21)

In [51]: s.__dict__
Out[51]: {'name': 'hui', '_age': 21}

In [52]: s2 = Student('jack', 20)

In [53]: s2.__dict__
Out[53]: {'name': 'jack', '_age': 20}

In [65]: # __str__
    ...: class Foo(object):
    ...:     pass
    ...:

In [66]: f = Foo()

In [67]: print(f)
<__main__.Foo object at 0x00000210E2715608>

In [68]: class Foo(object):
    ...:
    ...:     def __str__(self):
    ...:         return '< Custom Foo object str >'
    ...:

In [69]: f = Foo()

In [70]: print(f)
< Custom Foo object str >

# __getitem__、__setitem__、__delitem__
class MyDict(object):

    def __init__(self):
        self.my_dict = dict()

    def __getitem__(self, key):
        print('__getitem__() ', key)
        return self.my_dict.get(key, None)

    def __setitem__(self, key, value):
        print('__setitem__() ', key, value)
        self.my_dict.update(key=value)

    def __delitem__(self, key):
        print('__delitem__() ', key)
        del self.my_dict[key]


# ipython 测验        
In [33]: mdict = MyDict()

In [34]: print(mdict['name'])
__getitem__()  name
None

In [35]: # 新增

In [36]: mdict['name'] = 'hui'
__setitem__()  name hui

In [37]: mdict['age'] = 21
__setitem__()  age 21

In [38]: mdict['name']
__getitem__()  name
Out[38]: 'hui'

In [39]: mdict['age']
__getitem__()  age
Out[39]: 21

In [40]: # 更新

In [41]: mdict['name'] = 'jack'
__setitem__()  name jack

In [42]: mdict['name']
__getitem__()  name
Out[42]: 'jack'

In [43]: # 删除

In [44]: del mdict['age']
__delitem__()  age

In [45]: print(mdict['age'])
__getitem__()  age
None

# 切片操作
class MyList(object):

    def __init__(self):
        self.mlist = list()

    def __getitem__(self, index):
        print('__getitem__() called')
        print(index)
        if isinstance(index, slice):
            return self.mlist[index]

    def __setitem__(self, index, value):
        print('__getitem__() called')
        print(index, value)
        if isinstance(index, slice):
            self.mlist[index] = value

    def __delitem__(self, index):
        print('__delitem__() called')
        if isinstance(index, slice):
            del self.mlist[index]
     
    
# ipython 测验
In [70]: mlist = MyList()

In [71]: mlist[0]
__getitem__() called
0

In [72]: mlist[0:-1]
__getitem__() called
slice(0, -1, None)
Out[72]: []

In [73]: mlist[:] = [1,2,3]
__getitem__() called
slice(None, None, None) [1, 2, 3]

In [74]: mlist[:]
__getitem__() called
slice(None, None, None)
Out[74]: [1, 2, 3]

In [75]: mlist[0:2]
__getitem__() called
slice(0, 2, None)
Out[75]: [1, 2]

In [76]: mlist[::-1]
__getitem__() called
slice(None, None, -1)
Out[76]: [3, 2, 1]

In [77]: mlist[0]
__getitem__() called
0

In [78]: mlist[0:1]
__getitem__() called
slice(0, 1, None)
Out[78]: [1]

In [79]: del mlist[0:1]
__delitem__() called

In [80]: mlist[:]
__getitem__() called
slice(None, None, None)
Out[80]: [2, 3]

with open('foo.txt') as bar:
    # do something with bar
    pass

__enter__(self):

__exit__(self, exception_type, exception_value, traceback):

__copy__(self):

__deepcopy__(self, memodict={}):