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Python笔记(4)类(上)属性与描述符

时间:2016-07-17 11:44:15      阅读:359      评论:0      收藏:0      [点我收藏+]

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部分参考自:http://www.geekfan.net/7862/

新式类与经典类

2和3不一样,3都是新式类。

新式类和经典类的区别:

class A:
    #classic class
    """this is class A"""
    pass
    __slots__=(‘x‘,‘y‘)
    def test(self):
        # classic class test
        """this is A.test()"""
        print "A class"
class B(object):
    #new class
    """this is class B"""
    __slots__=(‘x‘,‘y‘)
    pass
    def test(self):
        # new class test
        """this is B.test()"""
        print "B class"

        
if __name__ == ‘__main__‘:
    a=A()
    b=B()
    print dir(a)
    print dir(b)
[‘__doc__‘, ‘__module__‘, ‘__slots__‘, ‘test‘]
[‘__class__‘, ‘__delattr__‘, ‘__doc__‘, ‘__format__‘, ‘__getattribute__‘, ‘__hash__‘, ‘__init__‘, ‘__module__‘, ‘__new__‘, ‘__reduce__‘, ‘__reduce_ex__‘, ‘__repr__‘, ‘__setattr__‘, ‘__sizeof__‘, ‘__slots__‘, ‘__str__‘, ‘__subclasshook__‘, ‘test‘, ‘x‘, ‘y‘]

  新式类要指明父类,上面代码class B 声明他的父类为object。

python是动态语言,可以动态的添加属性。

>>> a.x = 1
>>> a
<__main__.A instance at 0x05BBB620>
>>> a.x
1

 __slots__槽,属性限制了实例b只能添加x,y属性,a是经典类,可以继续添加,但是b是新式类不能继续添加。

>>> a.z = 2
>>> a.z
2
>>> b.z = 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: ‘B‘ object has no attribute ‘z‘

  

>>> help(a)
Help on instance of A in module __main__:

class A
 |  this is class A
 |  
 |  Methods defined here:
 |  
 |  test(self)
 |      this is A.test()

>>> help(b)
Help on B in module __main__ object:

class B(__builtin__.object)
 |  this is class B
 |  
 |  Methods defined here:
 |  
 |  test(self)
 |      this is B.test()
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  x
 |  
 |  y

  B类由于是新式类 __slots__起作用了,尽量使用新式类,因为这样python2,3中都能跑。

属性和封装

实例类型

__init__  双下划线是特殊方法,__init__定义实例属性,owner,country是实例属性,country是类属性。

调用的时候,比如类属性和实例属性名字一样,调用实例属性。如果没有实例属性,则去寻找是不是存在类属性。

class Car(object):
    country = u‘中国‘ 
    def __init__(self,owner=None):
        self.owner = owner
        self.country = "china"

if __name__ == ‘__main__‘:
    a = Car(u‘小张‘)
    print a.country
    
    a.country = u‘美国‘    
    print a.country
    print "--------------------------"
    del a.country
    print a.country
>>> china
美国
--------------------------
中国
私有属性

私有属性只在函数内部可见。通过get,set方法对其赋值更改。

在变量前加两个下划线__ 可以间接访问,只加一个下划线_模块私有化。变量前后各两个下划线__是系统自带的属性。

class Car(object):
    def __init__(self,owner=None):
        self.__owner = owner

    def getOwner(self):
        return self.__owner
    def setOwner(self, value):
        self.__owner = value

if __name__ == ‘__main__‘:
    a = Car(u‘黑板客‘)
    print a.getOwner()

  

>>> a.owner
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: ‘Car‘ object has no attribute ‘owner‘
>>> a.getOwner()
u‘\u9ed1\u677f\u5ba2‘
>>> dir(a)
[‘_Car__owner‘, ‘__class__‘, ‘__delattr__‘, ‘__dict__‘, ‘__doc__‘, ‘__format__‘, ‘__getattribute__‘, ‘__hash__‘, ‘__init__‘, ‘__module__‘, ‘__new__‘, ‘__reduce__‘, ‘__reduce_ex__‘, ‘__repr__‘, ‘__setattr__‘, ‘__sizeof__‘, ‘__str__‘, ‘__subclasshook__‘, ‘__weakref__‘, ‘getOwner‘, ‘setOwner‘]
>>> a._Car__owner
u‘\u9ed1\u677f\u5ba2‘

  

描述符

装饰器描述符

@property @xx.setter @xx.deleter

可以把get,set方法变成属性访问。

 

class Car(object):
    def __init__(self,owner=None):
        self._owner = owner
 
    @property
    def owner(self):
        return self._owner
    @owner.setter
    def owner(self, value):
        self._owner = value
    @owner.deleter
    def owner(self):
        self._owner = None
    
if __name__ == ‘__main__‘:
    a = Car(u‘你大爷‘)
    print a.owner
    del a.owner
    print a.owner 
你大爷
None

 这样一个owner get,set,del要定义三个,如果有别的属性,则又需要三个,这样会产生冗余,重复代码。

__getattr__, __setattr__, __delattr__

__getattr__  在变量的__dict__和_class__.__dict__中没有找到属性,就会调用__getattr__,如果有的话,就直接调用__dict__中的值了。

__setattr__ 变量赋值

__delattr__ 删除变量

class Car(object):
    country = u‘中国‘
    #__slots__=(‘length‘,‘width‘,‘height‘,‘owner‘,‘__dict__‘)
    
    def __init__(self, length, width, height, owner=None):
        self.owner = owner
        self.length = length
        self.width = width
        self.height = height
        
    def __getattr__(self,name):
        print "__getattr__",name
        return self.__dict__.get(name,None)
        
    def __setattr__(self,name,value):
        print "__setattr__",name
        if name!=‘owner‘:
            assert value>0, name+" must larger than 0"
        self.__dict__[name]=value
        
    def __delattr__(self,name):
        print "__delattr__",name
        if name==‘owner‘:
            self.__dict__[name]=None

            
if __name__ == ‘__main__‘:
    a = Car(1.2,1.4,1.5,u‘二大爷‘)

输出:  

__setattr__ owner
__setattr__ length
__setattr__ width
__setattr__ height

 把__slots__加上之后,因为可以访问__setattr__所以还是可以任意的加属性而不会报错,要使得slots有效果,得在__setattar__里面修改代码:

    def __getattr__(self,name):
        print "__getattr__",name
        assert name in self.__slots__, "Not have this attribute "+name
        return self.__dict__.get(name,None)
        
    def __setattr__(self,name,value):
        print "__setattr__",name
        assert name in self.__slots__, "Not have this attribute "+name        
        if name!=‘owner‘:
            assert value>0, name+" must larger than 0"
        self.__dict__[name]=value
        
    def __delattr__(self,name):
        print "__delattr__",name
        assert name in self.__slots__, "Not have this attribute "+name
        if name==‘owner‘:
类描述符

  描述符可以用作类的属性,数据描述符__get__,__set__,__del__。

class PositiveNum(object):
    def __init__(self):
        self.default = 1
        self.data = {}
 
    def __get__(self, instance, owner):
        # instance = x
        # owner = type(x)
        print "__get__",instance,owner
        return self.data.get(instance, self.default)
 
    def __set__(self, instance, value):
        # instance = x
        print "__set__",instance,value
        try:
            assert int(value)>0
            self.data[instance] = value
        except AssertionError:
            print "ERROR: "+str(value)+" is not positive number."
        except:
            print "ERROR: "+str(value)+" is not number value."
            
    def __delete__(self,instance):
        print "__delete__",instance
        del self.data[instance]
        
class Car(object):
    country = u‘中国‘
    length = PositiveNum()
    width = PositiveNum()
    height = PositiveNum()
    __slots__=(‘owner‘,‘length‘,‘width‘,‘height‘)
    
    def __init__(self, length, width, height, owner=None):
        self.owner = owner
        self.length = length
        self.width = width
        self.height = height
        

if __name__ == ‘__main__‘:
    a = Car(1.2,1.4,1.5,u‘黑板客‘)
    b = Car(2.2,2.4,2.5,u‘小明‘)
    print a.length
    a.length=1

  当解释器遇到print a.length时,它会把length当作一个带有__get__方法的描述符,调用a.length.__get__方法并将方法的返回值打印,这和上面的property相似。__get__接收两个参数:instance 实例对象,这里就是a.length中的a,另一个是实例的类型Car。在一些文档中,Car被称作描述符的所有者(owner)。如果需要访问Car.length,python将会调用Car.length.__get__(None,Car)。可以看到第一个参数要么是实例,要么是None。

  当解释器看到a.length = 1时,Python识别出length是一个带__set__方法的描述符,于是就调用a.length.__set__(a,100),第一个参数instance是实例,第二个是赋值。

  删除时Car.length.__delete__(a)。

  每个PositiveNum维护着一个字典,其中保存着所有者实例和对应数据的映射关系。调用a.length时,__get__方法会找出与a相关的数据,并发挥结果,如果不存在就返回一个默认值。__set__采用的方式相同,但是会包含额外的非法检查。

  描述符作用与类的层次上,每一个类的实例都共享同一个描述符。所以不同的实例对象不得不手动的管理不同的状态,需要显示的将参数精确的传递给__get__,__set__以及__delete__方法。

  如果将PositiveNum中的 data = {}去掉,由于描述符是基于类层面的,他们会共享同一个类属性,这就是使用字典的原因。__get__,__set__参数也指明哪一个实例,以实例为字典的key。

错误示例:

class PositiveNum(object):
    def __init__(self,value):
        self.val = value
 
    def __get__(self, instance, owner):
        # instance = a,b
        # owner = Car
        print "__get__",instance,owner
        return self.val
 
    def __set__(self, instance, value):
        # instance = a,b
        print "__set__",instance,value
        try:
            assert int(value)>0
            self.val = value
        except AssertionError:
            print "ERROR: "+str(value)+" is not positive number."
        except:
            print "ERROR: "+str(value)+" is not number value."  
            
    def __delete__(self,instance):
        print "__delete__",instance
        self.val = None

    #def __getattribute__(self,name):
        #print self, name
        
class Car(object):
    country = u‘中国‘
    length = PositiveNum(0)
    width = PositiveNum(0)
    height = PositiveNum(0)
    #__slots__=(‘owner‘,‘length‘,‘width‘,‘height‘)
    
    def __init__(self, length, width, height, owner=None):
        self.owner = owner
        self.length = length
        self.width = width
        self.height = height
        


if __name__ == ‘__main__‘:
    a = Car(1.2,1.4,1.5,u‘黑板客‘)
    b = Car(2.2,2.4,2.5,u‘小明‘)

  

a.length
__get__ <__main__.Car object at 0x098E61B0> <class ‘__main__.Car‘>
Out[39]: 2.2

b.length
__get__ <__main__.Car object at 0x098E6230> <class ‘__main__.Car‘>
Out[40]: 2.2

 虽然a定义的1.2,但由于与b公用一个类属性,所以也变成了2.2。

Python笔记(4)类(上)属性与描述符

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原文地址:http://www.cnblogs.com/zephyr-1/p/5665525.html

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