Python常见魔法方法

Python常见魔法方法

__new__

• 创建实例
• 在实例化过程中先调用__new__创建实例，再调用__init__初始化实例
• __new__参数是类本身，返回的是创建的实例

class Py(object):
    def __new__(cls, *args, **kwargs):
        print 'new'
        new_object = super(Py, cls).__new__(cls, *args, **kwargs)
        print new_object
        return new_object

    def __init__(self, *args, **kwargs):
        print 'init'
        self.name = 'lisi'
        
test = Py()
print test.name
#====输出====
new
<__main__.Py object at 0x000000000396FAC8>
init
lisi

__init__

• 实例初始化，给刚创建的实例增加属性，见__new__

__del__

• 实例生命周期结束时调用，删除实例

class Py(object):
    def __new__(cls, *args, **kwargs):
        print 'new'
        return super(Py, cls).__new__(cls, *args, **kwargs)
    def __init__(self):
        print 'init'
        self.name = 'lisi'
    def __del__(self):
        print 'delete object'
        
test = Py()
#====输出====
new
<__main__.Py object at 0x000000000361FBA8>
init
delete object

__setattr__

• 设置、修改实例属性时调用

class Py(object):
    def __init__(self):
        self.name = 'lisi'
        
    def __setattr__(self, key, value):
        print '属性名:{}, 属性值:{}'.format(key, value)
        super(Py, self).__setattr__(key, value)
test = Py()
test.name = 'zhangsan'
#====输出====
属性名:name, 属性值:lisi
属性名:name, 属性值:zhangsan

__getattribute__

• 访问实例属性、方法时调用
• item 为属性名或方法名

class Py(object):
    def __init__(self):
        self.name = 'lisi'
        
    def __getattribute__(self, item):
        print '__getattribute__'
        print item
        return super(Py, self).__getattribute__(item)
    
    def func(self):
        return 'func_str'
    
test = Py()
print test.name
print test.func()
#====输出====
__getattribute__
lisi
lisi
__getattribute__
func
func_str

• 尽量不要重写，防止进入无限循环，如下：name != ‘a’,会访问self.age, 访问self.age属性会先访问__getattribute__,如此进入循环

class Py(object):
    def __init__(self):
        self.name = 'lisi'
        self.age = 12
        
    def __getattribute__(self, item):
        print '__getattribute__'
        if item == 'a':
            return super(Py, self).__getattribute__(item)
        else:
            return self.name
test = Py()
print self.name

__getattr__

• 当访问不存在的属性、方法时先调用__getattribute__再调用__getattr
• 抛出 AttributeError

class Py(object):        
    def __getattr__(self, item):
        return '__getattr__，属性or方法不存在'
    
    def __getattribute__(self, item):
        print '__getattribute__'
        return super(Py, self).__getattribute__(item)
    
test = Py()
print test.age
#====输出====
__getattribute__
__getattr__，属性or方法不存在

__delattr__

• 删除一个实例对象的属性时调用

class Py(object):
    def __init__(self):
        self.name = 'lisi'

    def __delattr__(self, item):
        print '__delattr__', item
        return super(Py, self).__delattr__(item)
    
test = Py()
del test.name
#====输出====
__delattr__ name

__str__

• print 对象时返回的值

class Py(object):
    def __init__(self):
        self.name = 'lisi'

    def __str__(self):
        return '__str__'
    
test = Py()
print test
#====输出====
__str__

__call__

• 对象被当做方法调用时调用的方法

class Py(object):
    def __init__(self):
        self.name = 'lisi'

    def __call__(self, *args, **kwargs):
        return '__call__'
    
test = Py()
print test()
#====输出====
__call__

__dict__

• 打印对象所有属性和值

class Py(object):
    def __init__(self):
        self.name = 'lisi'
        self.age = 12
print(Py().__dict__)

#====输出====
{'name': 'lisi', 'age': 12}

setattr(), getattr(), hasattr()函数

object 类或对象
当 object 为对象时:
setattr == __setattr__, 
getattr == __getattr__,
delattr == __delattr__,


# 设置、重置对象属性值
setattr(object, name, value)  

# 获取属性或方法，返回属性值/方法地址
getattr(object, 'func/attr')

# 判断是否存在类或方法，返回 True/False
hasattr(object, 'func/attr') 

# 删除方法，属性
delattr(object, 'func/attr')