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Intfprgm helps separate interface from implementation.

Intfprgm offers a set of decorators to clearly mark and enforce a class to be
an interface, an abstract class, or a concrete class, and a function to be
abstract.

To define an abstract function, we can use ``abstract`` decorator::

    @abstract
    def func():
        pass

Or raise ``NotImplementedError`` in the very first line::

    def func():
        raise NotImplementedError()

An interface can be defined with ``interface`` decorator::

    @interface
    class Interface(object):

        def method_1(self):
            raise NotImplementedError()

        @abstract
        def method_2(self):
            pass

An abstract class is defined with the same ``abstract`` decorator::

    @abstract
    class BaseClass(Interface):

        def method_1(self):
            pass

        # method_2 is inherited from Interface and is still undefined

A concrete class is defined with ``concrete`` decorator::

    @concrete
    class ConcreteClass(BaseClass):

        def method_2(self):
            pass

When you need to ensure that your derived classes correctly override a method
in their bases, you can use ``overrides`` decorator. This decorator must be
applied on both the derived class, and its overriden methods::

    class Base(object):

        def to_be_overriden(self):
            pass


    @overrides
    class Derived(Base):

        @overrides
        def to_be_overriden(self):
            pass