CODEWITHSUNDEEP
pythonpython-2.7inheritancerecursionsuper
flakes
flakes

Reputation: 23624

Circular inheritance? What?

So I've been messing around in python and I don't understand what the shell is going on with this.

I start by writing these classes in this order.. comments are my understanding of what's happening:

# class A1 inherits object
class A(object):
    pass

# class B1 inherits A1, object
class B(A):
    def a(self):
        print "b",
        super(B, self).a()

# class A2 inherits B1, A1, object
class A(B):
    def a(self):
        print "a",
        super(A, self).a()

# class B2 inherits A2, B1, A1, object
class B(A):
    def a(self):
        print "b",
        super(B, self).a()

When I create an instance of A which is A2 and call A.a,

>>> a = A()
>>> a.a()
a b

Traceback (most recent call last):
  File "<pyshell#263>", line 1, in <module>
    a.a()
  File "<pyshell#258>", line 4, in a
    super(A, self).a()
  File "<pyshell#256>", line 4, in a
    super(B, self).a()
TypeError: super(type, obj): obj must be an instance or subtype of type

I thought the error would be (as Kevin suggests) AttributeError: A instance has no attribute 'a' because I expected A2 > B1 > A1 (no a function) Crash

Even more curious,. when I now create an instance of B, even more unexpected results happens..

>>> b = B()
>>> b.a()
b a b a b a b a b a b a b a b a b a b a b a b a b a b a b a b a b a b a b a b a ...

Shouldn't this crash the same way A did, but just one function more? What?

Upvotes: 3

Views: 2871

Answers (1)

flakes
flakes

Reputation: 23624

After creating class A2 and class B2, the lines 

super(B, self).a()
super(A, self).a()

Use the type A2 and B2 respectively for all class functions, A2.a,B1.a,B2.a..

This means that when I call A2.a, the super calls the base class B1.a. In B1.a the function attempts to use the super function with type B2 and a instance of A2.. thus giving a obj must be an instance or subtype of type error.

When I call B2.a, the super calls A2.a then calls B1.a where upon calling super with type B2 and an instance of B2 once again calls the A2.a function... Where it will loop indefinitely until a recursion error is thrown!

So it ends up only looking like a circular inheritance, when really its just two circular functions given a particular parameter.

Upvotes: 1

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