Why not just use a keyword argument for this?

That “just” makes the problem seem less complicated than it really is! :D

Let’s say you’re using a framework like falcon that doesn’t provide DI support out of the box. Assuming you wanted to

1. be able to swap out the implementation of todo manager easily in your tests and
2. avoid global state and mocking,

then your API might look something like this:

from falcon import API

# Define Database and TodoManager here...

class TodosResource:
  def __init__(self, todo_manager):
    self.todo_manager = todo_manager

  def on_get(self, req, resp):
    resp.media = self.todo_manager.get_all()


def setup_app(db_impl=Database, todo_manager_impl=TodoManager):
    db = db_impl()
    todo_manager = todo_manager_impl(db)

    app = API()
    app.add_route("/todos", TodosResource(todo_manager))
    return app

# In your entrypoint:
app = setup_app()

# In your tests:
app = setup_app(db_impl=StubDatabase)

This isn’t bad and it’s explicit, but manually configuring the dependency tree can get quite hairy over time and when a particular component’s dependencies change, rather than only changing that component’s code to depend on different deps, you’ll have to also change all of the places where the dependency tree is constructed (not a huge deal if you have this sort of setup_app function like I’ve got above, but not a particularly fun thing to do either way). Another downside here is that it’s not immediately clear what components the TodosResource.get method relies on from its header.

If, instead, you use type annotations to do the wiring for you then you get the best of all worlds in exchange for a little bit of magic (that, in my experience, people get used to fairly quickly).

I hope that makes sense!