Binding a method to another class

Question

class A(object):

    def a(self, b=1):
        print 'Up'
    d = {1 : a}

    def b( self ):
        print self.d[1]
        print self.b
        print self.d[1].__get__( self, A )()
        # print self.d[1]()

class B( object ):


    def a( self ):
        print 'here??'
        return 10000
    d = {1 : a}
    def b( self ):
        print 'hurray'

o = A()
o.b()

b = B()
type( o ).__dict__['b'].__get__( b, type( b ) )()

Hi Folks,

I was going through Python: Bind an Unbound Method? and http://users.rcn.com/python/download/Descriptor.htm and trying to experiment on my learning.

But, I have hit some new doubts now:-

In the last line of my code, I'm able to use __get__ with b object and instance: type(b). This only works if method b is defined in class B. Why is it so?
Even though the last line requires me to provide a method b in class B, still the method b in class A gets called. Why is it so?
To my utter surprise, after the above step, I notice that the method a of class A is not called by the code of method b of class A; instead, it calls the method a of class B. Why is it so?

I'm quite confused after seeing this behaviour. I might also need to learn more on descriptors. But, it would be a great help if you could answer my doubts

score 2 · Accepted Answer · answered Apr 29 '14 at 12:14

In the last line of my code, I'm able to use __get__ with b object and instance: type(b). This only works if method b is defined in class B. Why is it so?

You have to define a method b in class B, because in A.b you have print self.b. Here, self is an instance of the B class, so self.b means "the b method belonging to this B", not "the b method belonging to the class that this method exists in". If you delete print self.b, then the code will work even if B has no b.

Even though the last line requires me to provide a method b in class B, still the method b in class A gets called. Why is it so?

A.b is being called because you are explicitly accessing it with type( o ).__dict__['b']. Whether you bind that method to an A instance or a B instance doesn't matter; it's still A.b.

To my utter surprise, after the above step, I notice that the method a of class A is not called by the code of method b of class A; instead, it calls the method a of class B. Why is it so?

Even though b belongs to the class A, the self you pass to it is still an instance of the B class. Any attributes you access on that self will be B attributes, and any methods you call on it will be B methods.

Thanks Kevin. But, can yout plz explain what could be a real-life example in which we could use `type( o ).__dict__['b'].__get__( b, type( b ) )()` instead of `o.b.__get__( b, type( b ) )()` ? In other words, is binding the method `b` of `class A` to `class B` ever possible ? If yes, how and what are the practical use cases of that ? — GodMan, Apr 29 '14 at 12:36
"is binding the method b of class A to class B ever possible?" That's exactly what you're doing in your code. Getting `self` to pretend that it's an instance of A is something else entirely. In any case, I don't think there are any practical uses of this. Good OO code shouldn't try to "trick" the environment into thinking an object belongs to a class that it doesn't actually belong to. — Kevin, Apr 29 '14 at 13:23

Binding a method to another class

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