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 withaugments
.
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)