Improve wording in docs

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Roman Mogylatov 2022-04-16 22:29:19 -04:00
parent a0bb7c4ede
commit 9bc11a7828
2 changed files with 51 additions and 52 deletions

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@ -11,24 +11,24 @@ Dependency injection and inversion of control in Python
feature for testing or configuring project in different environments and explains
why it's better than 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
Originally dependency injection pattern got popular in languages with static typing like Java.
Dependency injection is a principle that helps to achieve an inversion of control. A
dependency injection framework can significantly improve the flexibility of a language
with static typing. Implementation of a dependency injection framework for a language
with 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
Python is an interpreted language with 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
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
This page describes the advantages of applying dependency injection in Python. It
contains Python examples that show how to implement dependency injection. It demonstrates the usage
of the ``Dependency Injector`` framework, its container, ``Factory``, ``Singleton``,
and ``Configuration`` providers. The example shows how to use providers' overriding feature
of ``Dependency Injector`` for testing or re-configuring a project in different environments and
explains why it's better than monkey-patching.
What is dependency injection?
@ -44,15 +44,14 @@ 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
- **High coupling**. If the coupling is high it's like using 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.
- **High cohesion**. High cohesion is like using screws. Quite easy to disassemble and
re-assemble in a different way. It is an opposite to high coupling.
Cohesion often correlates with coupling. Higher cohesion usually leads to lower coupling, and vice
versa.
Cohesion often correlates with coupling. Higher cohesion usually leads to lower coupling and vice versa.
Low coupling brings a flexibility. Your code becomes easier to change and test.
Low coupling brings flexibility. Your code becomes easier to change and test.
How to implement the dependency injection?
@ -150,14 +149,14 @@ Here comes the ``Dependency Injector``.
What does the Dependency Injector do?
-------------------------------------
With the dependency injection pattern objects loose the responsibility of assembling
With the dependency injection pattern, objects lose the responsibility of assembling
the dependencies. The ``Dependency Injector`` absorbs that responsibility.
``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
function argument. When you call this function, framework assembles and injects
the dependency.
.. code-block:: python
@ -198,79 +197,79 @@ the dependency.
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 you call the ``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.
When you do testing, you call the ``container.api_client.override()`` method 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
It also helps you in a re-configuring project for 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.
Objects assembling is consolidated in a container. Dependency injections are defined explicitly.
This makes it easier to understand and change how an application works.
Testing, Monkey-patching and dependency injection
-------------------------------------------------
The testability benefit is opposed to a monkey-patching.
The testability benefit is opposed to 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.
In Python, you can monkey-patch anything, anytime. The problem with monkey-patching is
that it's too fragile. The cause of it 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
With 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.
Also, monkey-patching is way too dirty to be used outside of the testing code for
re-configuring the project for the different environments.
Conclusion
----------
Dependency injection brings you 3 advantages:
Dependency injection provides you with three 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
- **Flexibility**. The components are loosely coupled. You can easily extend or change the
functionality of a system by combining the components in a different way. You even can do it on
the fly.
- **Testability**. Testing is easy because you can easily inject mocks instead of real objects
- **Testability**. Testing is easier 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
You have all the components and dependencies defined explicitly in a container. This
provides an overview and control of the application structure. It is easier to understand and
change it.
Is it worth to use a dependency injection in Python?
Is it worth applying 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.
larger the application the more significant the benefits.
Is it worth to use a framework for the dependency injection?
Is it worth using a framework for applying 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
lower than in 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
- Other engineers are familiar with it
Few advices at last:
An advice 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".
injection is just like "Wait, I need to state a need instead of getting something right away".
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
- **Common sense first**. Use common sense when applying dependency injection. It is a good
principle, but not a silver bullet. If you do it too much you will reveal too many of the
implementation details. Experience comes with practice and time.
What's next?
@ -304,8 +303,7 @@ Choose one of the following as a next step:
Useful links
------------
There are some useful links related to dependency injection design pattern
that could be used for further reading:
A few useful links related to a dependency injection design pattern for further reading:
+ https://en.wikipedia.org/wiki/Dependency_injection
+ https://martinfowler.com/articles/injection.html

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@ -10,11 +10,12 @@ follows `Semantic versioning`_
Development
-----------
- Update copyright year.
- Improve wording on the "Dependency injection and inversion of control in Python" docs page.
- Update typing in the main example and cohesion/coupling correlation definition in
"Dependency injection and inversion of control in Python".
Thanks to `@illia-v (Illia Volochii) <https://github.com/illia-v>`_ for the
PR (`#580 <https://github.com/ets-labs/python-dependency-injector/pull/580>`_).
- Update copyright year.
4.39.1
------