32. Dependency Injection

Here are 10 Python code snippets demonstrating dependency injection techniques that help with managing dependencies in Python applications to improve flexibility and testability:

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1. Basic Constructor Injection

class Database:
    def connect(self):
        return "Connected to the database."

class Service:
    def __init__(self, db: Database):
        self.db = db

    def get_data(self):
        return self.db.connect()

# Creating instances
db = Database()
service = Service(db)
print(service.get_data())

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2. Dependency Injection via Setter Method

class Database:
    def connect(self):
        return "Connected to the database."

class Service:
    def set_db(self, db: Database):
        self.db = db

    def get_data(self):
        return self.db.connect()

# Creating instances
db = Database()
service = Service()
service.set_db(db)
print(service.get_data())

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3. Dependency Injection with Function Injection

def service_function(db: Database):
    return db.connect()

class Database:
    def connect(self):
        return "Connected to the database."

# Create the dependency instance and inject it
db = Database()
print(service_function(db))

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4. Using Dependency Injection with a Context Manager

from contextlib import contextmanager

class Database:
    def connect(self):
        return "Connected to the database."

@contextmanager
def service_context():
    db = Database()
    yield db
    # Cleanup if needed

with service_context() as db:
    print(db.connect())

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5. Dependency Injection via a Factory Function

class Database:
    def connect(self):
        return "Connected to the database."

class Service:
    def __init__(self, db: Database):
        self.db = db

    def get_data(self):
        return self.db.connect()

def create_service():
    db = Database()
    return Service(db)

# Creating and using the service
service = create_service()
print(service.get_data())

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6. Using Dependency Injection with a Simple DI Container

class Database:
    def connect(self):
        return "Connected to the database."

class Service:
    def __init__(self, db: Database):
        self.db = db

    def get_data(self):
        return self.db.connect()

class Container:
    def __init__(self):
        self._services = {}

    def register(self, name, instance):
        self._services[name] = instance

    def get(self, name):
        return self._services[name]

# Registering and using services
container = Container()
container.register("db", Database())
container.register("service", Service(container.get("db")))

service = container.get("service")
print(service.get_data())

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7. Using Dependency Injection with Class Inheritance

class Database:
    def connect(self):
        return "Connected to the database."

class Service:
    def __init__(self, db: Database):
        self.db = db

    def get_data(self):
        return self.db.connect()

class MyService(Service):
    def __init__(self, db: Database):
        super().__init__(db)

# Creating instances
db = Database()
service = MyService(db)
print(service.get_data())

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8. Using Dependency Injection with Abstract Base Classes

from abc import ABC, abstractmethod

class Database(ABC):
    @abstractmethod
    def connect(self):
        pass

class MySQLDatabase(Database):
    def connect(self):
        return "Connected to MySQL database."

class Service:
    def __init__(self, db: Database):
        self.db = db

    def get_data(self):
        return self.db.connect()

# Creating instances
db = MySQLDatabase()
service = Service(db)
print(service.get_data())

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9. Dependency Injection for Unit Testing

class Database:
    def connect(self):
        return "Connected to the database."

class Service:
    def __init__(self, db: Database):
        self.db = db

    def get_data(self):
        return self.db.connect()

# Mock Database class for testing
class MockDatabase:
    def connect(self):
        return "Mock database connection."

# Unit Test
def test_service():
    mock_db = MockDatabase()
    service = Service(mock_db)
    assert service.get_data() == "Mock database connection."

test_service()

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10. Using Dependency Injection with injector Library

from injector import inject, Injector, Module, provider

class Database:
    def connect(self):
        return "Connected to the database."

class Service:
    @inject
    def __init__(self, db: Database):
        self.db = db

    def get_data(self):
        return self.db.connect()

class MyModule(Module):
    @provider
    def provide_database(self) -> Database:
        return Database()

# Creating injector and running the application
injector = Injector([MyModule()])
service = injector.get(Service)
print(service.get_data())

Install injector with pip install injector.

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These examples show various ways to implement dependency injection in Python, including constructor injection, setter injection, factory methods, DI containers, and using popular libraries like injector. Dependency injection helps to decouple components and makes code more testable and flexible.