lagom 0.3.2

Lagom, a type based dependency injection container

Lagom - Dependency injection container

Usage

Everything in Lagom is based on types. To create an object you pass the type to the container:

container = Container()
some_thing = container[SomeClass]

Defining a singleton

container[SomeExpensiveToCreateClass] = SomeExpensiveToCreateClass("up", "left")

alternatively if you want to defer construction until it's needed:

container[SomeExpensiveToCreateClass] = Singleton(SomeExpensiveToCreateClass)

Defining a type that gets recreated every time

container[SomeClass] = lambda: SomeClass("down", "spiral")

if the type needs things from the container the lambda can take a single argument which is the container:

container[SomeClass] = lambda c: SomeClass(c[SomeOtherDep], "spinning")

Alias a concrete instance to an ABC

container[SomeAbc] = ConcreteClass

Partially bind a function

Apply a function decorator to any function.

@bind_to_container(container)
def handle_some_request(request: typing.Dict, game: Game):
    # do something to the game
    pass

This function can now be called omitting any arguments that the container knows how to build.

# we can now call the following. the game argument will automagically
# come from the container
handle_some_request(request={"roll_dice": 5})

Example

App setup

from abc import ABC
from dataclasses import dataclass

from lagom import Container

#--------------------------------------------------------------
# Here is an example of some classes your application may be built from


@dataclass
class DiceApiUrl:
    url: str


class RateLimitingConfig:
    pass


class DiceClient(ABC):
    pass


class HttpDiceClient(DiceClient):

    def __init__(self, url: DiceApiUrl, limiting: RateLimitingConfig):
        pass


class Game:
    def __init__(self, dice_roller: DiceClient):
        pass

#--------------------------------------------------------------
# Next we setup some definitions

container = Container()
# We need a specific url
container[DiceApiUrl] = DiceApiUrl("https://roll.diceapi.com")
# Wherever our code wants a DiceClient we get the http one
container[DiceClient] = HttpDiceClient

#--------------------------------------------------------------
# Now the container can build the game object

game = container[Game]

Testing without patching

Taking the container from above we can now swap out the dice client to a test double/fake. When we get an instance of the Game class it will have the new fake dice client injected in.

def some_test(container: Container):
    container[DiceClient] = FakeDice(always_roll=6)
    game_to_test = container[Game]
    # TODO: act & assert on something

Design Goals

• Everything should be done by type. No reliance on names.
• All domain code should remain unmodified. No special decorators.
• Make use of modern python features (3.7 at the time of creation)
• The API should expose sensible typing (for use in pycharm/mypy)