==================================================================== Dependency Injector - Dependency injection microframework for Python ==================================================================== *Dependency Injector* is a dependency injection microframework for Python. It was designed to be unified, developer-friendly tool that helps to implement dependency injection pattern in formal, pretty, Pythonic way. *Dependency Injector* framework key features are: + Easy, smart, pythonic style. + Obvious, clear structure. + Extensibility and flexibility. + High performance. + Memory efficiency. + Thread safety. + Documentation. + Semantic versioning. *Dependency Injector* providers are implemented as C extension types using Cython. Status ------ +---------------------------------------+--------------------------------------------------------------------------------------------------------------------+ | *PyPi* | .. image:: https://img.shields.io/pypi/v/dependency_injector.svg | | | :target: https://pypi.python.org/pypi/dependency_injector/ | | | :alt: Latest Version | | | .. image:: https://img.shields.io/pypi/l/dependency_injector.svg | | | :target: https://pypi.python.org/pypi/dependency_injector/ | | | :alt: License | +---------------------------------------+--------------------------------------------------------------------------------------------------------------------+ | *Python versions and implementations* | .. image:: https://img.shields.io/pypi/pyversions/dependency_injector.svg | | | :target: https://pypi.python.org/pypi/dependency_injector/ | | | :alt: Supported Python versions | | | .. image:: https://img.shields.io/pypi/implementation/dependency_injector.svg | | | :target: https://pypi.python.org/pypi/dependency_injector/ | | | :alt: Supported Python implementations | +---------------------------------------+--------------------------------------------------------------------------------------------------------------------+ | *Builds and tests coverage* | .. image:: https://travis-ci.org/ets-labs/python-dependency-injector.svg?branch=master | | | :target: https://travis-ci.org/ets-labs/python-dependency-injector | | | :alt: Build Status | | | .. image:: https://coveralls.io/repos/ets-labs/python-dependency-injector/badge.svg | | | :target: https://coveralls.io/r/ets-labs/python-dependency-injector | | | :alt: Coverage Status | +---------------------------------------+--------------------------------------------------------------------------------------------------------------------+ | *Github* | .. image:: https://img.shields.io/github/watchers/ets-labs/python-dependency-injector.svg?style=social&label=Watch | | | :target: https://github.com/ets-labs/python-dependency-injector | | | :alt: Github watchers | | | .. image:: https://img.shields.io/github/stars/ets-labs/python-dependency-injector.svg?style=social&label=Star | | | :target: https://github.com/ets-labs/python-dependency-injector | | | :alt: Github stargazers | | | .. image:: https://img.shields.io/github/forks/ets-labs/python-dependency-injector.svg?style=social&label=Fork | | | :target: https://github.com/ets-labs/python-dependency-injector | | | :alt: Github forks | +---------------------------------------+--------------------------------------------------------------------------------------------------------------------+ Dependency injection -------------------- `Dependency injection`_ is a software design pattern that implements `Inversion of control`_ for resolving dependencies. Formally, if object **A** depends on object **B**, object **A** must not create or import object **B** directly. Instead of this object **A** must provide a way for *injecting* object **B**. The responsibilities of objects creation and dependencies injection are delegated to external code - the *dependency injector*. Popular terminology of dependency injection pattern: + Object **A**, that is dependant on object **B**, is often called - the *client*. + Object **B**, that is a dependency, is often called - the *service*. + External code that is responsible for creation of objects and injection of dependencies is often called - the *dependency injector*. There are several ways of how *service* can be injected into the *client*: + by passing it as ``__init__`` argument (constructor / initializer injection) + by setting it as attribute's value (attribute injection) + by passing it as method's argument (method injection) Dependency injection pattern has few strict rules that should be followed: + The *client* delegates to the *dependency injector* the responsibility of injecting its dependencies - the *service(s)*. + The *client* doesn't know how to create the *service*, it knows only interface of the *service*. The *service* doesn't know that it is used by the *client*. + The *dependency injector* knows how to create the *client* and the *service*, it also knows that the *client* depends on the *service*, and knows how to inject the *service* into the *client*. + The *client* and the *service* know nothing about the *dependency injector*. Dependency injection pattern provides the following advantages: + Control on application structure. + Decreased coupling between application components. + Increased code reusability. + Increased testability. + Increased maintainability. + Reconfiguration of system without rebuilding. Example of dependency injection ------------------------------- Let's go through next example: .. image:: https://raw.githubusercontent.com/wiki/ets-labs/python-dependency-injector/img/engines_cars/diagram.png :width: 100% :align: center Listing of ``example.engines`` module: .. code-block:: python """Dependency injection example, engines module.""" class Engine(object): """Example engine base class. Engine is a heart of every car. Engine is a very common term and could be implemented in very different ways. """ class GasolineEngine(Engine): """Gasoline engine.""" class DieselEngine(Engine): """Diesel engine.""" class ElectroEngine(Engine): """Electro engine.""" Listing of ``example.cars`` module: .. code-block:: python """Dependency injection example, cars module.""" class Car(object): """Example car.""" def __init__(self, engine): """Initializer.""" self._engine = engine # Engine is injected Next example demonstrates creation of several cars with different engines: .. code-block:: python """Dependency injection example, Cars & Engines.""" import example.cars import example.engines if __name__ == '__main__': gasoline_car = example.cars.Car(example.engines.GasolineEngine()) diesel_car = example.cars.Car(example.engines.DieselEngine()) electro_car = example.cars.Car(example.engines.ElectroEngine()) While previous example demonstrates advantages of dependency injection, there is a disadvantage demonstration as well - creation of car requires additional code for specification of dependencies. Nevertheless, this disadvantage could be easily avoided by using a dependency injection framework for creation of inversion of control container (IoC container). Example of creation of several inversion of control containers (IoC containers) using *Dependency Injector*: .. code-block:: python """Dependency injection example, Cars & Engines IoC containers.""" import example.cars import example.engines import dependency_injector.containers as containers import dependency_injector.providers as providers class Engines(containers.DeclarativeContainer): """IoC container of engine providers.""" gasoline = providers.Factory(example.engines.GasolineEngine) diesel = providers.Factory(example.engines.DieselEngine) electro = providers.Factory(example.engines.ElectroEngine) class Cars(containers.DeclarativeContainer): """IoC container of car providers.""" gasoline = providers.Factory(example.cars.Car, engine=Engines.gasoline) diesel = providers.Factory(example.cars.Car, engine=Engines.diesel) electro = providers.Factory(example.cars.Car, engine=Engines.electro) if __name__ == '__main__': gasoline_car = Cars.gasoline() diesel_car = Cars.diesel() electro_car = Cars.electro() Dependency injection in action ------------------------------ Brief example below is a simplified version of inversion of control containters from one of the real-life applications. This example demonstrates usage of *Dependency Injector* inversion of control containers & providers for specifying all application components and their dependencies beetween each other in one module. Besides other listed above advantages, it gives a great opportunity to control & manage application's structure in one place. .. code-block:: python """Example of dependency injection in Python.""" import logging import sqlite3 import boto.s3.connection import example.main import example.services import dependency_injector.containers as containers import dependency_injector.providers as providers class Core(containers.DeclarativeContainer): """IoC container of core component providers.""" configuration = providers.Configuration('config') logger = providers.Singleton(logging.Logger, name='example') class Gateways(containers.DeclarativeContainer): """IoC container of gateway (API clients to remote services) providers.""" database = providers.Singleton(sqlite3.connect, Core.configuration.database.dsn) s3 = providers.Singleton(boto.s3.connection.S3Connection, Core.configuration.aws.access_key_id, Core.configuration.aws.secret_access_key) class Services(containers.DeclarativeContainer): """IoC container of business service providers.""" users = providers.Factory(example.services.UsersService, db=Gateways.database, logger=Core.logger) auth = providers.Factory(example.services.AuthService, db=Gateways.database, logger=Core.logger, token_ttl=Core.configuration.auth.token_ttl) photos = providers.Factory(example.services.PhotosService, db=Gateways.database, s3=Gateways.s3, logger=Core.logger) class Application(containers.DeclarativeContainer): """IoC container of application component providers.""" main = providers.Callable(example.main.main, users_service=Services.users, auth_service=Services.auth, photos_service=Services.photos) Next example demonstrates run of example application defined above: .. code-block:: python """Run example application.""" import sys import logging from containers import Core, Application if __name__ == '__main__': # Configure platform: Core.configuration.update({'database': {'dsn': ':memory:'}, 'aws': {'access_key_id': 'KEY', 'secret_access_key': 'SECRET'}, 'auth': {'token_ttl': 3600}}) Core.logger().addHandler(logging.StreamHandler(sys.stdout)) # Run application: Application.main(uid=sys.argv[1], password=sys.argv[2], photo=sys.argv[3]) You can get more *Dependency Injector* examples in ``/examples`` directory on GitHub: https://github.com/ets-labs/python-dependency-injector Installation ------------ *Dependency Injector* library is available on `PyPi`_:: pip install dependency_injector Documentation ------------- *Dependency Injector* documentation is hosted on ReadTheDocs: - `User's guide`_ - `API docs`_ Feedback & Support ------------------ Feel free to post questions, bugs, feature requests, proposals etc. on *Dependency Injector* GitHub Issues: https://github.com/ets-labs/python-dependency-injector/issues Your feedback is quite important! .. _Dependency injection: http://en.wikipedia.org/wiki/Dependency_injection .. _Inversion of control: https://en.wikipedia.org/wiki/Inversion_of_control .. _PyPi: https://pypi.python.org/pypi/dependency_injector .. _User's guide: http://python-dependency-injector.ets-labs.org/ .. _API docs: http://python-dependency-injector.ets-labs.org/api/