Using super with a class method

Sometimes texts have to be read more for the flavor of the idea rather than for the details. This is one of those cases.

In the linked page, Examples 2.5, 2.6 and 2.7 should all use one method, do_your_stuff. (That is, do_something should be changed to do_your_stuff.)

In addition, as Ned Deily pointed out, A.do_your_stuff has to be a class method.

class A(object):    @classmethod    def do_your_stuff(cls):        print 'This is A'class B(A):    @classmethod    def do_your_stuff(cls):        super(B, cls).do_your_stuff()B.do_your_stuff()

super(B, cls).do_your_stuffreturns a bound method (see footnote 2). Since cls was passed as the second argument to super(), it is cls that gets bound to the returned method. In other words, cls gets passed as the first argument to the method do_your_stuff() of class A.

To reiterate: super(B, cls).do_your_stuff() causes A's do_your_stuff method to becalled with cls passed as the first argument. In order for that to work, A'sdo_your_stuff has to be a class method. The linked page doesn't mention that,but that is definitively the case.

PS. do_something = classmethod(do_something) is the old way of making a classmethod.The new(er) way is to use the @classmethod decorator.

Note that super(B, cls) can not be replaced by super(cls, cls). Doing so could lead to infinite loops. For example,

class A(object):    @classmethod    def do_your_stuff(cls):        print('This is A')class B(A):    @classmethod    def do_your_stuff(cls):        print('This is B')        # super(B, cls).do_your_stuff()  # CORRECT        super(cls, cls).do_your_stuff()  # WRONGclass C(B):    @classmethod    def do_your_stuff(cls):        print('This is C')        # super(C, cls).do_your_stuff()  # CORRECT        super(cls, cls).do_your_stuff()  # WRONGC.do_your_stuff()

will raise RuntimeError: maximum recursion depth exceeded while calling a Python object.

If cls is C, then super(cls, cls) searches C.mro() for the class that comes after C.

In [161]: C.mro()Out[161]: [__main__.C, __main__.B, __main__.A, object]

Since that class is B, when cls is C, super(cls, cls).do_your_stuff() always calls B.do_your_stuff. Since super(cls, cls).do_your_stuff() is called inside B.do_your_stuff, you end up calling B.do_your_stuff in an infinite loop.

In Python3, the 0-argument form of super was added so super(B, cls) could be replaced by super(), and Python3 will figure out from context that super() in the definition of class B should be equivalent to super(B, cls).

But in no circumstance is super(cls, cls) (or for similar reasons, super(type(self), self)) ever correct.

In Python 3, you can skip specifying arguments for super,

class A:    @classmethod    def f(cls):        return "A's f was called."class B(A):    @classmethod    def f(cls):        return super().f()assert B.f() == "A's f was called."

I've updated the article to make it a bit clearer: Python Attributes and Methods # Super

Your example using classmethod above shows what a class method is - it passes the class itself instead of the instance as the first parameter. But you don't even need an instance to call the method, for e.g.:

>>> class A(object):...     @classmethod...     def foo(cls):...         print cls... >>> A.foo() # note this is called directly on the class<class '__main__.A'>