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What is ``Dependency Injector``?
================================

``Dependency Injector`` is a dependency injection framework for Python.

It helps you implement the dependency injection principle.

What is dependency injection?
-----------------------------

Dependency injection is a principle that helps to decrease coupling and increase cohesion. Your
code becomes more flexible, clear and it is easier to test it.

How to implement dependency injection?
--------------------------------------

Objects do not create each other anymore. They provide a way to inject the needed dependency
instead.

Before:

.. code-block:: python

   class ApiClient:

       def __init__(self):
           self.api_key = os.getenv('API_KEY')
           self.timeout = os.getenv('TIMEOUT')


   class Service:

       def __init__(self):
           self.api_client = ApiClient()

After:

.. code-block:: python

   class ApiClient:

       def __init__(self, api_key: str, timeout: int):
           self.api_key = api_key
           self.timeout = timeout


   class Service:

       def __init__(self, api_client: ApiClient):
           self.api_client = api_client

Who creates the objects now? Look at the next section.

What does Dependency Injector do?
---------------------------------

``Dependency Injector`` provides you with the container and the providers that help you build
your application objects when you apply dependency injection principle:

.. code-block:: python

    from dependency_injector import containers, providers


    class ApiClient:

        def __init__(self, api_key: str, timeout: int):
            self.api_key = api_key
            self.timeout = timeout


    class Service:

        def __init__(self, api_client: ApiClient):
            self.api_client = api_client


    class Container(containers.DeclarativeContainer):

        config = providers.Configuration()

        api_client = providers.Singleton(
            ApiClient,
            api_key=config.api_key,
            timeout=config.timeout,
        )

        service = providers.Factory(
            Service,
            api_client=api_client,
        )


    if __name__ == '__main__':
        container = Container()
        container.config.from_yaml('config.yml')

        service = container.service()

        assert isinstance(service.api_client, ApiClient)

`More examples <https://github.com/ets-labs/python-dependency-injector/tree/master/examples>`_

Installation
------------

The package is available on the `PyPi`_::

    pip install dependency-injector

Documentation
-------------

The documentation is available on the `Read The Docs <http://python-dependency-injector.ets-labs.org/>`_

Tutorials
---------

Choose one of the following:

- `Flask web application tutorial <http://python-dependency-injector.ets-labs.org/tutorials/flask.html>`_
- `Aiohttp REST API tutorial <http://python-dependency-injector.ets-labs.org/tutorials/aiohttp.html>`_
- `Asyncio monitoring daemon tutorial <http://python-dependency-injector.ets-labs.org/tutorials/asyncio-daemon.html>`_
- `CLI application tutorial <http://python-dependency-injector.ets-labs.org/tutorials/cli.html>`_

Concept
-------

``Dependency Injector`` stands on two principles:

- Explicit is better than implicit (PEP20).
- Do no magic to your code.

How does it different from the other frameworks?

- **No autowiring.** The framework does NOT do any autowiring / autoresolving of the dependencies. You need to specify everything explicitly. Because *"Explicit is better than implicit" (PEP20)*.
- **Does not pollute your code.** Your application does NOT know and does NOT depend on the framework. No ``@inject`` decorators, annotations, patching or any other magic tricks.

``Dependency Injector`` makes a simple contract with you:

- You tell the framework how to build you code
- The framework does it for you

The power of the ``Dependency Injector`` is in its simplicity and straightforwardness. It is a simple tool for the powerful concept.

Frequently asked questions
--------------------------

What is the dependency injection?
 - dependency injection is a principle that decreases coupling and increases cohesion

Why should I do the dependency injection?
 - your code becomes more flexible, testable and clear
 - you have no problems when you need to understand how it works or change it 😎 

How do I start doing the dependency injection?
 - you start writing the code following the dependency injection principle
 - you register all of your application components and their dependencies in the container
 - when you need a component, you get it from the container

Why do I need a framework for this?
 - you need the framework for this to not create it by your own
 - this framework gives you the container and the providers
 - the container is like a dictionary with the batteries 🔋 
 - the providers manage the lifetime of your components, you will need factories, singletons, smart config object etc

What price do I pay and what do I get?
 - you need to explicitly specify the dependencies in the container
 - it will be extra work in the beginning
 - it will payoff when project grows or in two weeks 😊 (when you forget what project was about)

What features does the framework have?
 - building objects graph
 - smart configuration object
 - providers: factory, singleton, thread locals registers, etc
 - positional and keyword context injections
 - overriding of the objects in any part of the graph

What features the framework does NOT have?
 - autowiring / autoresolving of the dependencies
 - the annotations and ``@inject``-like decorators

Have a question?
 - Open a `Github Issue <https://github.com/ets-labs/python-dependency-injector/issues>`_

Found a bug?
 - Open a `Github Issue <https://github.com/ets-labs/python-dependency-injector/issues>`_

Want to help?
 - |star| Star the ``Dependency Injector`` on the `Github <https://github.com/ets-labs/python-dependency-injector/>`_
 - |new| Start a new project with the ``Dependency Injector``
 - |tell| Tell your friend about the ``Dependency Injector``

Want to contribute?
 - |fork| Fork the project
 - |pull| Open a pull request to the ``develop`` branch

.. _PyPi: https://pypi.org/project/dependency-injector/

.. |star| unicode:: U+2B50 U+FE0F .. star sign1
.. |new| unicode:: U+1F195 .. new sign
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.. |pull| unicode:: U+2B05 U+FE0F .. pull sign