Enable decorating a service type

[abdulhaq-e]

I have a library where I wrote something very similar to svcs but not as polished of course :D. I'm planning to remove my hacky dependency resolver code and replace it with svcs. However, looking at the source code and examples, there doesn't seem to be a way of "decorating" a service, i.e. registering it once then when registering it a second time, it can have access to the first instance.

Am I right with this conclusion? I can attempt to open a PR with my solution if that is the case.

[abdulhaq-e]

import svcs
import uuid
from typing import Protocol

class UserService(Protocol):
    pass

class UserSQLAlchemyService:
   pass

registry = svcs.Registry()

registry.register_factory(UserService, lambda: UserSQLAlchemyService)

class UserSQLAlchemyAdvancedService:
   def __init__(self, service: UserService):
        self.service = service

def user_sqlalchemy_service_decorator(svcs_container):
    original_service = svcs_container.get(UserService)
    return UserSQLAlchemyAdvancedService(original_service)

registry.register_factory(UserService, user_sqlalchemy_service_decorator)

Something like the above, I'm relying on a plugin architecture where decorating existing services is important. In my implementation, the the decoration has a special api and they are all stored in an iterable where they are applied after all "main" registrations are resolved.

So something like this would be possible:

registry.register_factory(UserService, user_sqlalchemy_service_decorator, decorator=True)

or to be more explicit:

registry.decorate_factory(UserService, user_sqlalchemy_service_decorator)