Simple Dependency Injection with decorators

registry.py

# encoding: utf-8
"""
Dependency injection made simple

    @requires('stores')
    class Action(object):
        self.stores = None

    @provides('stores')
    class InMemoryStores(Stores):
        pass

    action = SomeAction()
    assert isinstance(action.stores, InMemoryStores)

When first Action is instantiated, this will trigger instantiation of 'stores'
providing class. This class constructor cannot take any arguments.
"""

__all__ = 'requires', 'provides'


def requires(name):
    def decorator(cls):
        init = cls.__init__
        def wrapper(self, *args, **kwargs):
            setattr(self, name, _get_instance(name))
            init(self, *args, **kwargs)
        cls.__init__ = wrapper
        return cls
    return decorator

def provides(name):
    def decorator(factory):
        _factories[name] = factory
        return factory
    return decorator


_factories = {}
_instances = {}

def _get_instance(name):
    try:
        return _instances[name]
    except KeyError:
        return _create_instance(name)

def _create_instance(name):
    try:
        instance = _factories[name]()
        _instances[name] = instance
        return instance
    except KeyError:
        raise StandardError('Nothing registered for name: %s' % name)