I Promise

This repository provides a Python base class, and various decorators for specifying promises relating to inheritance. It provides three inheritance patterns:

• implementing,
• overriding, and
• augmenting.

Base class

Checking promises depends on inheritance from the base class AbstractBaseClass. Unlike the standard library's similar class abc.ABCMeta, AbstractBaseClass does not bring in any metaclasses. This is thanks to Python 3.6's PEP 487, which added __init_subclass__.

Implementing

Implementing is the pattern whereby an inheriting class's method implements an abstract method from a base class method. It is declared using the decorators:

• abc.abstractmethod from the standard library, and
• implements, which indicates that a method implements an abstract method in a base class

For example:

class HasAbstractMethod(AbstractBaseClass):

    @abstractmethod
    def f(self):
        raise NotImplementedError


class ImplementsAbstractMethod(HasAbstractMethod):

    @implements(HasAbstractMethod)
    def f(self):
        return 0

Overriding

Overriding is the pattern whereby an inheriting class's method replaces the implementation of a base class method. It is declared using the decorator overrides, which marks the overriding method.

An overriding method could call super, but does not have to:

class HasRegularMethod(AbstractBaseClass):

    def f(self):
        return 1


class OverridesRegularMethod(HasRegularMethod):

    @overrides(HasRegularMethod)
    def f(self):
        return 2

Augmenting

Augmenting is a special case of overriding whereby the inheriting class's method not only overrides the base class method, but extends its functionality. This means that it must delegate to super in all code paths. This pattern is typical in multiple inheritance.

We hope that Python linters will be able to check for the super call.

Augmenting is declared using two decorators:

• augments indicates that this method must call super within its definition and thus augments the behavior of the base class method, and
• must_agugment indicates that child classes that define this method must decorate their method overriddes with augments.

For example:

class HasMustAugmentMethod(AbstractBaseClass):

    @must_augment
    def f(self):
        # must_augment prevents this behavior from being lost.
        self.times_f_called += 1
        return 0


class AugmentsMethod(HasMustAugmentMethod):

    @augments(HasMustAugmentMethod)
    def f(self, extra=0, **kwargs):
        return super().f(**kwargs) + extra


class AugmentsMethodFurther(AugmentsMethod):

    @augments(HasMustAugmentMethod)
    def f(self, **kwargs):
        print("f has been called")
        return super().f(**kwargs)