python解释器如何决定何时调用”set”和”get”方法，何时不调用？

How does python interpreter decides when to invokes the set and get methods and when not to?

本问题已经有最佳答案，请猛点这里访问。

假设我们定义下面的python描述符

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class FooDescriptor():
def __init__(self, name='foo', initval='initval'):
self.name = name
self.val = initval

def __get__(self, instance, cls):
print("invoke get {} in instance {} in class {}".format(self.name, instance, cls))
return self.val

def __set__(self, instance, val):
print("invoke set {} in instance {}".format(self.name, instance))
self.val = val

现在，如果我们将类Foo定义为具有类型FooDescriptor的类属性和相同类型的实例属性：

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class Foo():
cdescr = FooDescriptor(name='class attribute descr', initval=1)
def __init__(self):
self.idescr = FooDescriptor(name='instance attribute descr', initval=2)

我们得到类描述符所需的行为：

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>>> foo = Foo()
>>> foo.cdescr
invoke get class attribute descr in instance <__main__.Foo object at 0x10189ecc0> in class <class '__main__.Foo'>
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>>> foo.cdescr = 100
invoke set class attribute descr in instance <__main__.Foo object at 0x10189ecc0>
>>> foo.cdescr
invoke get class attribute descr in instance <__main__.Foo object at 0x10189ecc0> in class <class '__main__.Foo'>
100

但是，对于实例属性idescr，当访问或设置属性时，描述符对象的__set__和__get__方法不会被调用：

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>>> foo.idescr
<__main__.FooDescriptor object at 0x10189ecf8>
>>> foo.idescr = 120
>>> foo.idescr
120
>>> type(foo.idescr)
<class 'int'>

如果能更好地理解描述符的范围以及当属性protocal .出现时，python如何决定调用它们的__set__和__get__，我们将不胜感激。

How does python interpreter decides when to invokes the __set__ and __get__ methods and when not to?

How does python interpreter decides when to invokes the set and get methods and when not to?