This library is a trivial implementation of an interface in Python, with the following aspects / features:
- It fails at import time, not at construction, so you know immediately when you have a problem.
- It's quite simple (very few LOC) and lenient where it counts
- It exclusively uses decorators, so...
- It does not require inheritance (reducing 'forced' subclassing)
- It does not enforce any typing checks
- It is intended to 'enhance' duck typing by avoiding common pitfalls (forgot to implement something on your fake duck class, overwrote something fundamental, etc.)
Given a simple interface like:
@interfaces.define class DuckInterface(object): @interfaces.require def quack(self): """All Ducks must implement a 'quack' method.""" pass
...the following will raise a MissingRequiredAttribute exception at import time:
@interfaces.implement(DuckInterface) class Silent(object): # no quack method pass
This, however works:
@interfaces.implement(DuckInterface) class Tree(object): @interfaces.final def quack(self): return "the tree appears to quack." tree = Tree() tree.quack()
Additionally, if you are interested in using the
final method decorator
outside of an interface, you can do so using the
strict class decorator
around any class you want to check:
@interfaces.strict class BaseClass(object): @interfaces.final def method(self): return "Old functionality!" # the following will raise an exception at import: @interfaces.strict class SubClass(BaseClass): def method(self): return "New functionality!"
It will also check to make sure your arguments match in your implemented methods. For example:
@interfaces.define class Base(object): @interfaces.require def foo(self, a, b, *args): pass @interfaces.implements(Base) class Sub(object): def foo(self, a): pass
...will raise an
InvalidMethodSignature error. It checks arguments (and
argument names) as well as the presence of args and *kwargs.
Obviously, this does nothing for enforcing interfaces on the calling side. I've contemplated something like:
def get_json(instance): # raises an exception if instance does not implement # JSONViewInterface interfaces.expects(instance, JSONViewInterface) return instance.to_json()
...but that's just plain type checking, and the only thing you gain over isinstance is a more explicit exception at runtime. Feedback welcome.