def singleton(cls):
    instances = {}

    def get_instance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]

    return get_instance


@singleton
class Package(object):
    def register(self, name, module):
        setattr(self, name, module)


def inject(*modules):
    def wrapper(cls):
        for module in modules:
            setattr(cls, module, getattr(Package(), module))
        return cls
    return wrapper


def register(name):
    def wrapper(cls):
        configurations = {
            'database': (
                ('mysql', ),
                {'host': 'localhost', 'user': 'user', 'password': 'password'}
            )
        }

        if name in configurations:
            args, kwargs = configurations[name]
        else:
            args, kwargs = [], {}

        Package().register(name, cls(*args, **kwargs))

        return cls
    return wrapper


@register("database")
class Database(object):
    def __init__(self, db_type, host=None, user=None, password=None):
        self.db_type = db_type
        self.host = host

    def __str__(self):
        return self.db_type + " @ " + self.host


@register("metric")
@inject("database")
class Metric:
    def test(self):
        print self.database

    def __str__(self):
        return "metric instance"


@inject("database", "metric")
class Analyzer:
    def test(self):
        print self.database
        print self.metric


#instance0 = Database()

instance1 = Metric()
instance1.test()

instance2 = Analyzer()
instance2.test()