python class 常用内置方法分析

class ABC(object):
    def __init__(self):
        self.value = {}
        
    def __setattr__(self, name, val):
        """ x.__setattr__('name', val) <==> x.name = val """
        print 'call __setattr__: name={0}, val={1}'.format(name, val)
        object.__setattr__(self, name, val)
        
    def __getattribute__(self, name):
        """ x.__getattribute__('name') <==> x.name """
        print 'call __getattribute__: name={0}'.format(name)
        return object.__getattribute__(self, name)
            
    def __getattr__(self, name):
        """
        called __getattr__ when call __getattribute__ raise AttributeError exception
        """
        print 'call __getattr__: name={0}'.format(name)
        return None
    
    def __delattr__(self, name):
        """ x.__delattr__('name') <==> del x.name """
        print 'call __delattr__: name={0}'.format(name)
        object.__delattr__(self, name)
    
    def __setitem__(self, name, val):
        """ x.__setitem__(name, val) <==> x[name]=val """
        print 'call __setitem__: name={0}, val={1}'.format(name, val)
        self.value[name] = val
        
    def __getitem__(self, name):
        """ x.__getitem__(name) <==> x[name] """
        print 'call __getitem__: name={0}'.format(name)
        try:
            return self.value[name]
        except KeyError:
            raise KeyError(name)
            
    def __delitem__(self, name):
        """ x.__delitem__(name) <==> del x[name] """
        print 'del __getitem__: name={0}'.format(name)
        del self.value[name]
        
    def __iter__(self):
        """ x.__iter__() <==> iter(x) """
        for item in self.value:
            yield item
        raise StopIteration

class ABC 从 object 继承，object 已经定义了 __setattr__，__getattribute__，__delattr__ 方法，这里重载只是打印日志，方便跟踪调用。

abc = ABC()
abc.x = 1

打印：

call __setattr__: name=value, val={}

call __setattr__: name=x, val=1

 

__setattr__ 方法调用了两次，因为实例化abc的时候会首先调用__init__ 方法，对value属性赋值，接着对x属性赋值

 

print abc.x

打印：

call __getattribute__: name=x

1

 

如果访问abc没有的属性y，__getattribute__将会抛出AttributeError异常，如果class ABC重载了__getattr__方法，__getattr__将会被调用，return None，否则__getattr__不会被调用。

print abc.y

打印：

call __getattribute__: name=y

call __getattr__: name=y

None

 

class ABC 定义了 __setitem__ __getitem__ __delitem__ 方法，支持 self[key] 操作，这几个方法object没有实现

 

abc['y'] = 2
print abc['y']

打印：

call __setitem__: name=y, val=2

call __getattribute__: name=value

call __getitem__: name=y

call __getattribute__: name=value

2

 

class ABC 定义了 __iter__ 方法，支持迭代操作

 

abc = ABC()
abc.x = 1
abc['y'] = 2
abc['z'] = 3
for item in abc:
    print item

打印：

call __getattribute__: name=value

y

z

 

class ABC 重载了 __delattr__ 并且定义了 __delitem__ 方法，支持 del abc.x，del abc[y]操作，删除不存在的属性和key将会抛出异常。

 

print abc.__dict__
del abc.x
del abc['y']
print abc.__dict__

打印：

{'x': 1, 'value': {'y': 2, 'z': 3}}

call __delattr__: name=x

del __getitem__: name=y

{'value': {'z': 3}}