Constructor Injection

Constructor Injection is a Dependency Injection technique where dependencies are provided through a class constructor. This is the most common and recommended form of DI.

How It Works

Instead of creating dependencies internally, a class declares them as constructor parameters:

class UserService:
    def __init__(self, database_service: DatabaseService):
        self.database_service = database_service
 
    def get_user(self, user_id):
        return self.database_service.fetch_user(user_id)

The caller is responsible for providing the dependency:

db = DatabaseService()
user_service = UserService(db)

Why It’s Preferred

• Explicit — Dependencies are visible in the signature
• Valid state — Object is fully initialized after construction
• Immutable — Dependencies can be set once and not changed

Other Injection Techniques

• Setter Injection — Dependencies provided through setter methods after construction. Useful when dependencies are optional.
• Interface Injection — The class implements an interface that defines an injection method (e.g., set_dependency(dep)). The injector calls this method to provide the dependency. Less common in practice.

TIP

Prefer constructor injection. Use setter injection only for optional dependencies.