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Dependency Injection

Properties1
tagspython, oop

Dependency injection means passing an object’s dependencies from the outside instead of constructing them internally. It makes code easier to test and change.

Constructor Injection

class RobotControls:
    def __init__(self, api):
        self.api = api
 
    def move_west(self):
        self.api.send_command('move_west', data={'meters': 1})

RobotControls depends on an object that has send_command. It does not need to know whether that object is a real API client, a mock, or a test double.

Benefit

The main benefit is substitutability. Tests can pass a fake dependency. Production code can pass the real dependency.

Cost

Constructor injection can create nested construction:

fence_guarding_robot = FenceGuardingRobot(RobotControls(RobotApi()))

When object graphs grow, dependency construction may need a factory or container.

Inheritance Is Not The Same

Using inheritance and super() can reduce constructor nesting, but it changes the relationship. Inheritance means “is a”; dependency injection means “uses a”.

This connects to Python Inheritance And Polymorphism because inheritance should preserve substitutability. When a class only needs to use another service, composition through dependency injection is often cleaner.

References

Backlinks