Python Multiple inheritance

Question

I have 3 classes A,B and D as given below

class A(object):
    def test(self):
        print "called A"

class B(object):
    def test(self):
        print "called B"

class D(A,B):
    def test(self):
        super(A,self).test()

inst_d=D()
inst_d.test()

----------------------------------------
Output:
  called B

Question: In D.test(), I am calling super(A,self).test(). Why is only B.test() called even though the method A.test() also exists?

The way Python's super works is somewhat non-obvious. A great practical guide on using it correctly is this one: rhettinger.wordpress.com/2011/05/26/super-considered-super — millimoose
– millimoose, Commented Jul 18, 2012 at 18:18

Daniel Roseman · Accepted Answer · 2012-07-18 18:15:46Z

6

Because you've told it not to. In D.test you've told it to call the test method of the parent of A - that's what super does.

Normally you want to use the current class name in the super call.

answered Jul 18, 2012 at 18:15

Daniel Roseman

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5 Comments

kindall Over a year ago

Also, the other classes need to also call super() to get the next class in the MRO.

millimoose Over a year ago

And "normally" might as well be "pretty much always"; a hypothetical use case for skipping to a random place in the method resolution order would probably be rather exotic.

Otto Allmendinger Over a year ago

The answer is a little unclear - B is not the parent of A, so it isn't obvious why B.test() is called.

Anoop Over a year ago

Thanks Daniel ,in this case mro will start from parent of A not from class A got it.

millimoose Over a year ago

@Anoop: Exactly. The first parameter to super() indicates where the currently executing method is in the MRO that built by starting at the type of the second parameter. Then the method called through the resulting proxy will be the next one in the MRO after the current one.

Marco de Wit · Accepted Answer · 2012-07-18 18:57:17Z

0

super(A,self).test() means : call the test method of the object after A in self's method resolution order (mro) .

using D.__mro__ you see the the method resolution order is:

<class '__main__.D'>, <class '__main__.A'>, <class '__main__.B'>, <type 'object'>

So test of B is called.

In Python 3 you only have to type super().test() and it does what you want. In Python 2 you need to type : super(D,self).test()

answered Jul 18, 2012 at 18:57

Marco de Wit

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Senthil Kumaran · Accepted Answer · 2012-07-18 19:09:09Z

0

Usually the super is called with the current class name and you let the MRO of python to take care which parent class it should call according to the algorithm it follows. So your code will look like this, for the behavior you intend.

class D(A,B):
    def test(self):
        super(D,self).test()

Note super(D,self).test()

answered Jul 18, 2012 at 19:09

Senthil Kumaran

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