Dependency injection and inversion of control in Python
=======================================================

.. meta::
   :keywords: Python,DI,Dependency injection,IoC,Inversion of Control,Example
   :description: This page describes a usage of the dependency injection and inversion of control
                 in Python. It contains Python examples that show how to implement dependency
                 injection. It demonstrates a usage of the dependency injection framework
                 Dependency Injector, its container, Factory, Singleton and Configuration
                 providers. The example show how to use Dependency Injector providers overriding
                 feature for testing or configuring project in different environments and explains
                 why it's better then monkey-patching.

Originally dependency injection pattern got popular in the languages with a static typing,
like Java. Dependency injection is a principle that helps to achieve an inversion of control.
Dependency injection framework can significantly improve a flexibility of the language
with a static typing. Implementation of a dependency injection framework for a language
with a static typing is not something that one can do quickly. It will be a quite complex thing
to be done well. And will take time.

Python is an interpreted language with a dynamic typing. There is an opinion that dependency
injection doesn't work for it as well as it does for Java. A lot of the flexibility is already
built in. Also there is an opinion that a dependency injection framework is something that
Python developer rarely needs. Python developers say that dependency injection can be implemented
easily using language fundamentals.

This page describes the advantages of the dependency injection usage in Python. It
contains Python examples that show how to implement dependency injection. It demonstrates a usage
of the dependency injection framework ``Dependency Injector``, its container, ``Factory``,
``Singleton`` and ``Configuration`` providers. The example shows how to use ``Dependency Injector``
providers overriding feature for testing or configuring project in different environments and
explains why it's better then monkey-patching.

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

Let's see what the dependency injection is.

Dependency injection is a principle that helps to decrease coupling and increase cohesion.

.. image:: images/coupling-cohesion.png

What is coupling and cohesion?

Coupling and cohesion are about how tough the components are tied.

- **High coupling**. If the coupling is high it's like using a superglue or welding. No easy way
  to disassemble.
- **High cohesion**. High cohesion is like using the screws. Very easy to disassemble and
  assemble back or assemble a different way. It is an opposite to high coupling.

When the cohesion is high the coupling is low.

Low coupling brings a flexibility. Your code becomes easier to change and test.

How to implement the dependency injection?

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

Before:

.. code-block:: python

   import os


   class ApiClient:

       def __init__(self):
           self.api_key = os.getenv('API_KEY')  # <-- dependency
           self.timeout = os.getenv('TIMEOUT')  # <-- dependency


   class Service:

       def __init__(self):
           self.api_client = ApiClient()  # <-- dependency


   def main() -> None:
       service = Service()  # <-- dependency
       ...


   if __name__ == '__main__':
       main()

After:

.. code-block:: python

   import os


   class ApiClient:

       def __init__(self, api_key: str, timeout: int):
           self.api_key = api_key  # <-- dependency is injected
           self.timeout = timeout  # <-- dependency is injected


   class Service:

       def __init__(self, api_client: ApiClient):
           self.api_client = api_client  # <-- dependency is injected


   def main(service: Service):  # <-- dependency is injected
       ...


   if __name__ == '__main__':
       main(
           service=Service(
               api_client=ApiClient(
                   api_key=os.getenv('API_KEY'),
                   timeout=os.getenv('TIMEOUT'),
               ),
           ),
       )

``ApiClient`` is decoupled from knowing where the options come from. You can read a key and a
timeout from a configuration file or even get them from a database.

``Service`` is decoupled from the ``ApiClient``. It does not create it anymore. You can provide a
stub or other compatible object.

Function ``main()`` is decoupled from ``Service``. It receives it as an argument.

Flexibility comes with a price.

Now you need to assemble and inject the objects like this:

.. code-block:: python

   main(
       service=Service(
           api_client=ApiClient(
               api_key=os.getenv('API_KEY'),
               timeout=os.getenv('TIMEOUT'),
           ),
       ),
   )

The assembly code might get duplicated and it'll become harder to change the application structure.

Here comes the ``Dependency Injector``.

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

With the dependency injection pattern objects loose the responsibility of assembling
the dependencies. The ``Dependency Injector`` absorbs that responsibilities.

``Dependency Injector`` helps to assemble and inject the dependencies.

It provides a container and providers that help you with the objects assembly.
When you need an object you place a ``Provide`` marker as a default value of a
function argument. When you call this function framework assembles and injects
the dependency.

.. code-block:: python

   from dependency_injector import containers, providers
   from dependency_injector.wiring import inject, Provide


   class Container(containers.DeclarativeContainer):

       config = providers.Configuration()

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

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


   @inject
   def main(service: Service = Provide[Container.service]):
       ...


   if __name__ == '__main__':
       container = Container()
       container.config.api_key.from_env('API_KEY')
       container.config.timeout.from_env('TIMEOUT')
       container.wire(modules=[sys.modules[__name__]])

       main()  # <-- dependency is injected automatically

       with container.api_client.override(mock.Mock()):
           main()  # <-- overridden dependency is injected automatically

When you call ``main()`` function the ``Service`` dependency is assembled and injected automatically.

When doing a testing you call the ``container.api_client.override()`` to replace the real API
client with a mock. When you call ``main()`` the mock is injected.

You can override any provider with another provider.

It also helps you in configuring project for the different environments: replace an API client
with a stub on the dev or stage.

Objects assembling is consolidated in the container. Dependency injections are defined explicitly.
This makes easier to understand and change how application works.

Testing, Monkey-patching and dependency injection
-------------------------------------------------

The testability benefit is opposed to a monkey-patching.

In Python you can monkey-patch
anything, anytime. The problem with a monkey-patching is that it's too fragile. The reason is that
when you monkey-patch you do something that wasn't intended to be done. You monkey-patch the
implementation details. When implementation changes the monkey-patching is broken.

With a dependency injection you patch the interface, not an implementation. This is a way more
stable approach.

Also monkey-patching is a way too dirty to be used outside of the testing code for
reconfiguring the project for the different environments.

Conclusion
----------

Dependency injection brings you 3 advantages:

- **Flexibility**. The components are loosely coupled. You can easily extend or change a
  functionality of the system by combining the components different way. You even can do it on
  the fly.
- **Testability**. Testing is easy because you can easily inject mocks instead of real objects
  that use API or database, etc.
- **Clearness and maintainability**. Dependency injection helps you reveal the dependencies.
  Implicit becomes explicit. And "Explicit is better than implicit" (PEP 20 - The Zen of Python).
  You have all the components and dependencies defined explicitly in the container. This
  provides an overview and control on the application structure. It is easy to understand and
  change it.

Is it worth to use a dependency injection in Python?

It depends on what you build. The advantages above are not too important if you use Python as a
scripting language. The picture is different when you use Python to create an application. The
larger the application the more significant is the benefit.

Is it worth to use a framework for the dependency injection?

The complexity of the dependency injection pattern implementation in Python is
lower than in the other languages but it's still in place. It doesn't mean you have to use a
framework but using a framework is beneficial because the framework is:

- Already implemented
- Tested on all platforms and versions of Python
- Documented
- Supported
- Known to the other engineers

Few advices at last:

- **Give it a try**. Dependency injection is counter-intuitive. Our nature is that
  when we need something the first thought that comes to our mind is to go and get it. Dependency
  injection is just like "Wait, I need to state a need instead of getting something right now".
  It's like a little investment that will pay-off later. The advice is to just give it a try for
  two weeks. This time will be enough for getting your own impression. If you don't like it you
  won't lose too much.
- **Common sense first**. Use a common sense when apply dependency injection. It is a good
  principle, but not a silver bullet. If you do it too much you will reveal too much of the
  implementation details. Experience comes with practice and time.

What's next?
------------

Choose one of the following as a next step:

- Look at the application examples:
    - :ref:`application-single-container`
    - :ref:`application-multiple-containers`
    - :ref:`decoupled-packages`
    - :ref:`django-example`
    - :ref:`flask-example`
    - :ref:`flask-blueprints-example`
    - :ref:`aiohttp-example`
    - :ref:`sanic-example`
    - :ref:`fastapi-example`
    - :ref:`fastapi-redis-example`
    - :ref:`fastapi-sqlalchemy-example`
- Pass the tutorials:
    - :ref:`flask-tutorial`
    - :ref:`aiohttp-tutorial`
    - :ref:`asyncio-daemon-tutorial`
    - :ref:`cli-tutorial`
- Know more about the ``Dependency Injector`` :ref:`key-features`
- Know more about the :ref:`providers`
- Know more about the :ref:`wiring`
- Go to the :ref:`contents`

Useful links
------------

There are some useful links related to dependency injection design pattern
that could be used for further reading:

+ https://en.wikipedia.org/wiki/Dependency_injection
+ https://martinfowler.com/articles/injection.html
+ https://github.com/ets-labs/python-dependency-injector
+ https://pypi.org/project/dependency-injector/

.. disqus::