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Improve wording in docs
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@ -11,24 +11,24 @@ Dependency injection and inversion of control in Python
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feature for testing or configuring project in different environments and explains
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why it's better than monkey-patching.
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Originally dependency injection pattern got popular in the languages with a static typing,
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like Java. Dependency injection is a principle that helps to achieve an inversion of control.
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Dependency injection framework can significantly improve a flexibility of the language
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with a static typing. Implementation of a dependency injection framework for a language
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with a static typing is not something that one can do quickly. It will be a quite complex thing
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Originally dependency injection pattern got popular in languages with static typing like Java.
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Dependency injection is a principle that helps to achieve an inversion of control. A
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dependency injection framework can significantly improve the flexibility of a language
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with static typing. Implementation of a dependency injection framework for a language
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with static typing is not something that one can do quickly. It will be a quite complex thing
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to be done well. And will take time.
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Python is an interpreted language with a dynamic typing. There is an opinion that dependency
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Python is an interpreted language with dynamic typing. There is an opinion that dependency
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injection doesn't work for it as well as it does for Java. A lot of the flexibility is already
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built in. Also there is an opinion that a dependency injection framework is something that
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built-in. Also, there is an opinion that a dependency injection framework is something that
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Python developer rarely needs. Python developers say that dependency injection can be implemented
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easily using language fundamentals.
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This page describes the advantages of the dependency injection usage in Python. It
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contains Python examples that show how to implement dependency injection. It demonstrates a usage
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of the dependency injection framework ``Dependency Injector``, its container, ``Factory``,
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``Singleton`` and ``Configuration`` providers. The example shows how to use ``Dependency Injector``
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providers overriding feature for testing or configuring project in different environments and
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This page describes the advantages of applying dependency injection in Python. It
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contains Python examples that show how to implement dependency injection. It demonstrates the usage
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of the ``Dependency Injector`` framework, its container, ``Factory``, ``Singleton``,
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and ``Configuration`` providers. The example shows how to use providers' overriding feature
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of ``Dependency Injector`` for testing or re-configuring a project in different environments and
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explains why it's better than monkey-patching.
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What is dependency injection?
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@ -44,15 +44,14 @@ What is coupling and cohesion?
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Coupling and cohesion are about how tough the components are tied.
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- **High coupling**. If the coupling is high it's like using a superglue or welding. No easy way
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- **High coupling**. If the coupling is high it's like using superglue or welding. No easy way
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to disassemble.
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- **High cohesion**. High cohesion is like using the screws. Very easy to disassemble and
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assemble back or assemble a different way. It is an opposite to high coupling.
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- **High cohesion**. High cohesion is like using screws. Quite easy to disassemble and
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re-assemble in a different way. It is an opposite to high coupling.
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Cohesion often correlates with coupling. Higher cohesion usually leads to lower coupling, and vice
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versa.
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Cohesion often correlates with coupling. Higher cohesion usually leads to lower coupling and vice versa.
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Low coupling brings a flexibility. Your code becomes easier to change and test.
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Low coupling brings flexibility. Your code becomes easier to change and test.
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How to implement the dependency injection?
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@ -150,14 +149,14 @@ Here comes the ``Dependency Injector``.
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What does the Dependency Injector do?
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-------------------------------------
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With the dependency injection pattern objects loose the responsibility of assembling
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With the dependency injection pattern, objects lose the responsibility of assembling
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the dependencies. The ``Dependency Injector`` absorbs that responsibility.
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``Dependency Injector`` helps to assemble and inject the dependencies.
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It provides a container and providers that help you with the objects assembly.
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When you need an object you place a ``Provide`` marker as a default value of a
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function argument. When you call this function framework assembles and injects
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function argument. When you call this function, framework assembles and injects
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the dependency.
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.. code-block:: python
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@ -198,79 +197,79 @@ the dependency.
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with container.api_client.override(mock.Mock()):
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main() # <-- overridden dependency is injected automatically
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When you call ``main()`` function the ``Service`` dependency is assembled and injected automatically.
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When you call the ``main()`` function the ``Service`` dependency is assembled and injected automatically.
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When doing a testing you call the ``container.api_client.override()`` to replace the real API
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client with a mock. When you call ``main()`` the mock is injected.
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When you do testing, you call the ``container.api_client.override()`` method to replace the real API
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client with a mock. When you call ``main()``, the mock is injected.
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You can override any provider with another provider.
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It also helps you in configuring project for the different environments: replace an API client
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It also helps you in a re-configuring project for different environments: replace an API client
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with a stub on the dev or stage.
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Objects assembling is consolidated in the container. Dependency injections are defined explicitly.
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This makes easier to understand and change how application works.
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Objects assembling is consolidated in a container. Dependency injections are defined explicitly.
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This makes it easier to understand and change how an application works.
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Testing, Monkey-patching and dependency injection
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-------------------------------------------------
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The testability benefit is opposed to a monkey-patching.
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The testability benefit is opposed to monkey-patching.
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In Python you can monkey-patch
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anything, anytime. The problem with a monkey-patching is that it's too fragile. The reason is that
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when you monkey-patch you do something that wasn't intended to be done. You monkey-patch the
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implementation details. When implementation changes the monkey-patching is broken.
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In Python, you can monkey-patch anything, anytime. The problem with monkey-patching is
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that it's too fragile. The cause of it is that when you monkey-patch you do something that
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wasn't intended to be done. You monkey-patch the implementation details. When implementation
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changes the monkey-patching is broken.
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With a dependency injection you patch the interface, not an implementation. This is a way more
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With dependency injection, you patch the interface, not an implementation. This is a way more
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stable approach.
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Also monkey-patching is a way too dirty to be used outside of the testing code for
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reconfiguring the project for the different environments.
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Also, monkey-patching is way too dirty to be used outside of the testing code for
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re-configuring the project for the different environments.
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Conclusion
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----------
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Dependency injection brings you 3 advantages:
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Dependency injection provides you with three advantages:
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- **Flexibility**. The components are loosely coupled. You can easily extend or change a
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functionality of the system by combining the components different way. You even can do it on
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- **Flexibility**. The components are loosely coupled. You can easily extend or change the
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functionality of a system by combining the components in a different way. You even can do it on
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the fly.
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- **Testability**. Testing is easy because you can easily inject mocks instead of real objects
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- **Testability**. Testing is easier because you can easily inject mocks instead of real objects
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that use API or database, etc.
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- **Clearness and maintainability**. Dependency injection helps you reveal the dependencies.
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Implicit becomes explicit. And "Explicit is better than implicit" (PEP 20 - The Zen of Python).
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You have all the components and dependencies defined explicitly in the container. This
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provides an overview and control on the application structure. It is easy to understand and
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You have all the components and dependencies defined explicitly in a container. This
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provides an overview and control of the application structure. It is easier to understand and
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change it.
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Is it worth to use a dependency injection in Python?
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Is it worth applying dependency injection in Python?
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It depends on what you build. The advantages above are not too important if you use Python as a
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scripting language. The picture is different when you use Python to create an application. The
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larger the application the more significant is the benefit.
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larger the application the more significant the benefits.
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Is it worth to use a framework for the dependency injection?
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Is it worth using a framework for applying dependency injection?
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The complexity of the dependency injection pattern implementation in Python is
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lower than in the other languages but it's still in place. It doesn't mean you have to use a
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lower than in other languages but it's still in place. It doesn't mean you have to use a
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framework but using a framework is beneficial because the framework is:
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- Already implemented
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- Tested on all platforms and versions of Python
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- Documented
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- Supported
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- Known to the other engineers
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- Other engineers are familiar with it
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Few advices at last:
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An advice at last:
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- **Give it a try**. Dependency injection is counter-intuitive. Our nature is that
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when we need something the first thought that comes to our mind is to go and get it. Dependency
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injection is just like "Wait, I need to state a need instead of getting something right now".
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injection is just like "Wait, I need to state a need instead of getting something right away".
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It's like a little investment that will pay-off later. The advice is to just give it a try for
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two weeks. This time will be enough for getting your own impression. If you don't like it you
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won't lose too much.
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- **Common sense first**. Use a common sense when apply dependency injection. It is a good
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principle, but not a silver bullet. If you do it too much you will reveal too much of the
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- **Common sense first**. Use common sense when applying dependency injection. It is a good
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principle, but not a silver bullet. If you do it too much you will reveal too many of the
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implementation details. Experience comes with practice and time.
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What's next?
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@ -304,8 +303,7 @@ Choose one of the following as a next step:
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Useful links
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------------
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There are some useful links related to dependency injection design pattern
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that could be used for further reading:
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A few useful links related to a dependency injection design pattern for further reading:
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+ https://en.wikipedia.org/wiki/Dependency_injection
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+ https://martinfowler.com/articles/injection.html
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@ -10,11 +10,12 @@ follows `Semantic versioning`_
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Development
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-----------
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- Update copyright year.
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- Improve wording on the "Dependency injection and inversion of control in Python" docs page.
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- Update typing in the main example and cohesion/coupling correlation definition in
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"Dependency injection and inversion of control in Python".
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Thanks to `@illia-v (Illia Volochii) <https://github.com/illia-v>`_ for the
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PR (`#580 <https://github.com/ets-labs/python-dependency-injector/pull/580>`_).
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- Update copyright year.
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4.39.1
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------
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