Python

Simplify Testing with Dependency Injection

Python clean code tip:

Use dependency injection to simplify testing

Example:

from dataclasses import dataclass

from fastapi import FastAPI


@dataclass
class User:
    username: str


class StartUserOnboarding:
    def __init__(self, user_repository):
        self._user_repository = user_repository

    def execute(self, username):
        user = User(username=username)
        self._user_repository.add(user)


class InMemoryUserRepository:
    def __init__(self):
        self._users = []

    def add(self, user):
        self._users.append(user)

    def get_by_username(self, username):
        return next(user for user in self._users if user.username == username)


class SQLiteUserRepository:
    def __init__(self, config):
        self._config = config

    def add(self, user):
        print(f"Running some SQL statements for insert DATABASE_PATH")

    def get_by_username(self, username):
        print(f"Running some SQL statements for fetch from {self._config.DATABASE_PATH}")


def test_user_is_added_to_repository():
    username = "[email protected]"
    repository = InMemoryUserRepository()
    use_case = StartUserOnboarding(user_repository=repository)

    use_case.execute(username)

    assert repository.get_by_username(username).username


class ApplicationConfig:
    DATABASE_PATH = "db"


app = FastAPI()


@app.post("/users/start-onboarding", status_code=202)
async def start_user_onboarding(username: str):
    StartUserOnboarding(SQLiteUserRepository(ApplicationConfig())).execute(username)

    return "OK"