Python常用魔法方法

Python常用魔法方法

1 、__new__()

__new__是在实例创建之前被调用的，用于创建实例，然后返回该实例对象

2、__init__()

__init__是当实例对象创建完成后被调用的，用于初始化一个类实例

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

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

# 执行
obj = MyClass('cc', 24)
print(obj.name)
print(obj.age)

# 输出
__new__ called
__init__ called
cc
24

3 、__str__()

对象被执行打印操作的时候自动触发, 该方法必须返回字符串类型的数据,很多时候用来更加精准的描述对象

class Myclass(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __str__(self):
        return '打印的变成我了'

# 执行
obj = Myclass('cc', 24)
print(obj)

# 输出
打印的变成我了

4、__call__()

对象被加括号调用的时候自动触发

class Myclass(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __call__(self, *args, **kwargs):        
        print('__call__ called', args, kwargs)
        return '嘿嘿嘿'

# 执行
obj = Myclass('cc', 24)
obj(1, 2, 3, sex='male')
print(obj(1, 2))

# 输出
__call__ called (1, 2, 3) {'sex': 'male'}
__call__ called (1, 2) {}
嘿嘿嘿

5、__del__()

对象被执行(被动、主动)删除操作之后自动执行

class Myclass(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age
       
    def __del__(self):
        print('__del__ called')

# 主动删除:使用del关键字主动删除
obj = Myclass('cc', 24)
del obj  # 主动删除对象
print(obj.name)

# 输出
NameError: name 'obj' is not defined
__del__ called

------------------------------------

# 被动删除:即对象调用完后自动删除
obj = Myclass('cc', 24)
print(obj.name)
print(obj.age)

# 输出
cc
24
__del__ called

6、__getattr__()

对象查找不存在名字的时候自动触发

class MyClass(object):
    def __init__(self, name, age):
        print('__init__ called')
        self.name = name
        self.age = age
        
    def __getattr__(self, item):
        print('__getattr__ called')
        return '找不到'

# 执行
obj = MyClass('cc', 24)
print(obj.name)
print(obj.age)
print(obj.sex)

# 输出
__init__ called
cc
24
__getattr__ called
找不到

7、__setattr__()
对象在执行添加属性操作的时候自动触发>>>obj.变量名=变量值

class Myclass(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __setattr__(self, key, value):
        print('__setattr__ called')
        print(key, value)

# 执行
obj = Myclass('cc', 24)
obj.sex = 'male'

# 输出
__setattr__ called
name cc
__setattr__ called
age 24
__setattr__ called
sex male

8、__getattribute__()
对象查找名字无论名字是否存在都会执行该方法
如果类中有'__getattribute__'方法 那么就'不会'去执行'__getattr__'方法

class Myclass(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __getattribute__(self, item):
        print('__getattribute__ called', item)

    def __getattr__(self, item):
        print('__getattr__ called', item)
        return '找不到'

# 执行
obj = Myclass('cc', 24)
print(obj.name)
print(obj.sex)

# 输出
__getattribute__ called name
None
__getattribute__ called sex
None

9、__enter__() 与 __exit__()

__enter__: 对象被执行with上下文管理语法自动触发

__exit__: 对象被执行with上下文管理语法结束之后自动触发

class Myclass(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __enter__(self):
        print('__enter__ called')

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('__exit__ called')

# 执行
obj = Myclass('cc', 24)
with obj as f:
    print('执行中...')
print(123)

# 输出
__enter__ called
执行中...
__exit__ called
123

作者：陈超