在Python中，如何在描述符上调用实例方法？

Question

Given a descriptor class MyDescriptor with an extra method describe , and a class that defines an attribute a using the descriptor, how can I call the method describe on a ?

class MyClass(object):
    a = MyDescriptor()

    def dump(self):
        print a.describe()

When I create an instance of MyClass, set its attribute a to 42 and then call dump() , I get:

"AttributeError: 'int' object has no attribute 'describe'".

I'd like to use the descriptor to carry some metadata about the way its set method was called. I need to find a way to ask the descriptor instance for that metadata.

Answer 1

To access the descriptor object itself you need to call it using class's __dict__ attribute.

def dump(self):
    print type(self).__dict__['a'].describe()

For more info on descriptors read Descriptor HowTo Guide .

Demo:

class MyDescriptor(object):

    def __get__(self, obj, type=None):
        return self._val

    def __set__(self, obj, val):
        self._val = val

    def describe(self):
        return 'inside describe'

class MyClass(object):

    a = MyDescriptor()

    def __init__(self):
        self.a = 42

    def dump(self):
        print 'Inside dump()'
        print self.a.describe()

    def dump2(self):
        print 'Inside dump2()'
        print type(self).__dict__['a'].describe()

MyClass().dump2()
print '-'*20
MyClass().dump()

Output:

Inside dump2()
inside describe
--------------------
Inside dump()

Traceback (most recent call last):
  File "C:\Python27\SO.PY", line 26, in <module>
    MyClass().dump()
  File "C:\Python27\SO.PY", line 19, in dump
    print self.a.describe()
AttributeError: 'int' object has no attribute 'describe'