描述符

Python 面向对象编程进阶

Jake Roach

Data Engineer

改变与属性的交互方式

class Student:
    def __init__(self, name, ssn):
        self.name = name
        self.ssn = ssn

# 创建一个学生对象，并访问 "ssn" 属性
shaw = Student("Daniel Shaw", "193-80-1821")
print(shaw.ssn)

# 若值被修改或删除，应如何处理？

193-80-1821

使用 @property 创建描述符

描述符是用于控制属性的获取、设置或删除方式的对象。

使用 @property 创建描述符

"getter"、"setter"、"deleter"
方法名 == 属性名

也可通过以下方式创建描述符…

property() 函数
__get__()、__set__()、__delete__()

class Student:
    def __init__(self, name, ssn):
        self.name = name
        self.ssn = ssn

    @property  
    def ssn(self):
        return "XXX-XX-" + self._ssn[-4:]

    @ssn.setter  
    def ssn(self, new_ssn):
        self._ssn = new_ssn 

    @ssn.deleter
    def ssn(self):
        raise AttributeError("Can't delete SSN")

@property

class Student:
    def __init__(self, name, ssn):
        self.name = name
        self.ssn = ssn  # Does not need to include an underscore

    @property  
    def ssn(self):  # This is the "getter" method
        return "XXX-XX-" + self._ssn[-4:]

控制 ssn 的读取方式
只与 self._ssn 交互，不与 self.ssn 交互

@ssn.setter

class Student:
    def __init__(self, name, ssn):
        self.name = name
        self.ssn = ssn
    ...

    @ssn.setter  
    def ssn(self, new_ssn):
        if len(new_ssn) == 11:
          # Add things such as data 
          # validation, operations on
          # other attributes, etc.
          self._ssn = new_ssn

@<attribute-name>.setter

ssn 的"setter"方法
校验数据质量
可操作其他属性

请记得只与 self._ssn 交互，而非 self.ssn！

@ssn.deleter

class Student:
    def __init__(self, name, ssn):
        self.name = name
        self.ssn = ssn

    ...

    @ssn.deleter
    def ssn(self):
        # Can perform clean up, soft delete, raise exception
        raise AttributeError("Can't delete SSN")

描述符实战

shaw = Student("Daniel Shaw", "193-80-1821")
print(shaw.ssn)  # Access the ssn attribute

XXX-XX-1821

shaw.ssn = "821-11-9380"  # Update Shaw's social security number
print(shaw.ssn)

XXX-XX-9380

del shaw.ssn  # Attempt to delete the ssn attribute

AttributeError: Can't delete SSN

Passons à la pratique !

Python 面向对象编程进阶