import functools
import weakref

class Property(object):
    instances = weakref.WeakValueDictionary()

    def __new__(cls, name):
        if name in Property.instances:
            return Property.instances[name]

        obj = super().__new__(cls)
        setattr(obj, 'name', name)

        Property.instances[name] = obj

        return obj


class requires(object):
    def __init__(self, *args, dynamic=False):
        self.requires = set(args)
        self.dynamic = dynamic

    def __call__(self, func, *args, **kwargs):
        @functools.wraps(func)
        def static_wrapper(*args, **kwargs):
            return func(*args, **kwargs)

        @functools.wraps(func)
        def dynamic_wrapper(*args, **kwargs):
            for req in self.requires:
                assert req in Property.instances, 'missing requirement: {}'.format(req)

            return func(*args, **kwargs)

        if self.dynamic:
            return dynamic_wrapper

        for req in self.requires:
            assert req in Property.instances

        return static_wrapper


class provides(object):
    def __init__(self, *args):
        self.properties = list(args)

    def __call__(self, func, *args, **kwargs):
        @functools.wraps(func)
        def impl(*args, **kwargs):
            return func(*args, **kwargs)

        setattr(impl, '__provides__', [Property(p) for p in self.properties])

        return impl


@requires('bar', dynamic=True)
@provides('foo')
class Foo(object):
    def stuff(self):
        print('foo')
        provides('baz')

    @requires('baz', dynamic=True)
    def other(self):
        print('baz')


@provides('bar')
class Bar(object):
    pass

f = Foo()
f.stuff()
f.other()