Interface-Based Programming in Python?

Question

In superclass in Python, how can I call the function that's overridden in its subclass?

class A:
    @staticmethod
    def interfaceBasedMethod():
        print "Want to delegate to overriding method in subclass"

    @staticmethod
    def a():
        interfaceBasedMethod() # <-- I know this causes error. But here 
                               # I want to call the overridden method in subclass.

class B(A):
    @staticmethod
    def interfaceBasedMethod():
        print "Class B processes."

if __name__ == "__main__":
    b=B
    b.a()

Ideal output:

Class B processes.

Google search doesn't really return pages about Interface-Based Programming in Python, although it should be common in Java etc. Thanks!

Why are you using `@staticmethod` if you're trying to do polymorphism? — Greg Hewgill, Oct 28 '11 at 02:09
@Brendan Long thanks. I updated the code and hope it looks better. I'm Java programmer and still new to python. — IsaacS, Oct 28 '11 at 02:10
[Read this. Now.](http://dirtsimple.org/2004/12/python-is-not-java.html) — Ignacio Vazquez-Abrams, Oct 28 '11 at 02:10
I thought I saw, at some point, a module/tool that aided with using "interfaces" in Python. That is, it helped to ensure that objects conformed to certain contracts (aka "interfaces"), but I can't recall the name or the scope of the module :( — , Oct 28 '11 at 02:35
@pst There are a couple -- I think one is from Enthought (Traits?). — agf, Oct 28 '11 at 02:40
@pst: This is not an interface problem though. The method is already defined; the interface won't help. — Ignacio Vazquez-Abrams, Oct 28 '11 at 02:56

score 2 · Accepted Answer · answered Oct 28 '11 at 02:16

Putting aside questions of style, use classmethod not staticmethod. It passes in the class which is being used.

class A:
    @classmethod
    def interfaceBasedMethod(cls):
        print "Want to delegate to overriding method in subclass"

    @classmethod
    def a(cls):
        cls.interfaceBasedMethod()

class B(A):
    @classmethod
    def interfaceBasedMethod(cls):
        print "Class B processes."

if __name__ == "__main__":
    b=B
    b.a()

score 1 · Answer 2 · edited May 23 '17 at 12:27

1

Your inheritance question was answered before here.

But in summary, Python provides a function super(type[, instance]).

Documented here, super provides a fairly clean way for accessing superclass methods from within a subclass. This page provides an exhaustive study of super() use cases, but this is the run-down.

class C(B):
    def method(self, arg):
        super(C, self).method(arg)

Which could be generalized a bit....

class C(B):
    def method(self, arg):
        super(self.__class__, self).method(arg)

edited May 23 '17 at 12:27

Community

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answered Oct 28 '11 at 02:22

arrdem

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This question is about polymorphism, not strictly inheritance. – Ignacio Vazquez-Abrams Oct 28 '11 at 02:25

Interface-Based Programming in Python?

2 Answers2