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Dependency Injector - Dependency injection microframework for Python
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*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
------

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| *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                                                                                                   |
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| *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                                                                          |
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|                                       |    :alt: Build Status                                                                                              |
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|                                       |    :alt: Github watchers                                                                                           |
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|                                       |    :alt: Github stargazers                                                                                         |
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|                                       |    :alt: Github forks                                                                                              |
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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/