python - 为什么在描述符类的 __get__ 中需要 "if instance is None"？

Question

我从 Effective Python item 31 中得到以下示例:

from weakref import WeakKeyDictionary
class Grade(object):
    def __init__(self):
        self._values = WeakKeyDictionary()
    def __get__(self, instance, instance_type):
        if instance is None: return self
        return self._values.get(instance, 0)

    def __set__(self, instance, value):
        if not (0 <= value <= 100):
            raise ValueError('Grade must be between 0 and 100')
        self._values[instance] = value


# Example 16
class Exam(object):
    math_grade = Grade()
    writing_grade = Grade()
    science_grade = Grade()

first_exam = Exam()
first_exam.writing_grade = 82
second_exam = Exam()
second_exam.writing_grade = 75
print('First ', first_exam.writing_grade, 'is right')
print('Second', second_exam.writing_grade, 'is right')

我想不出任何理由在 __get__ 中使用 if instance is None: return self。 Exam(或其他使用 Grade 的潜在类(class))实例如何成为 None？

Answer 1

Python 将在访问类上的描述符时为实例传入 None。

在这种情况下，通过返回 self，您可以访问类上的描述符对象，而无需绕过协议(protocol)(通过访问 ClassObj.__dict__['name_of_descriptor']) .

>>> class DemoDescriptor:
...     def __get__(self, instance, type_):
...         if instance is None:
...             print('Accessing descriptor on the class')
...             return self
...         print('Accessing descriptor on the instance')
...         return 'Descriptor value for instance {!r}'.format(instance)
... 
>>> class DemoClass(object):
...     foo = DemoDescriptor()
... 
>>> DemoClass.foo  # on the class
Accessing descriptor on the class
<__main__.DemoDescriptor object at 0x1041d3c50>
>>> DemoClass.__dict__['foo']  # bypassing the descriptor protocol
<__main__.DemoDescriptor object at 0x1041d3c50>
>>> DemoClass().foo  # on the instance
Accessing descriptor on the instance
'Descriptor value for instance <__main__.DemoClass object at 0x1041d3438>'

这也是 function 和 property 对象的 __get__ 方法实现的工作方式。

对于您的具体情况，Exam.math_grade、Exam.writing_grade 或 Exam.science_grade 都将调用 Grade.__get__ ，为实例传入 None，为 instance_type 传入 Exam。