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108 lines
4.0 KiB
ReStructuredText
108 lines
4.0 KiB
ReStructuredText
Singleton provider
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==================
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.. meta::
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:keywords: Python,DI,Dependency injection,IoC,Inversion of Control,Singleton,Pattern,Example,
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Threads,Multithreading,Scoped
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:description: Singleton provider helps to provide a single object. This page
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demonstrates how to use a Singleton provider. It also provides the example
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of using a singleton and thread locals singleton in the multi-threaded
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environment.
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.. currentmodule:: dependency_injector.providers
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:py:class:`Singleton` provider provides single object. It memorizes the first created object and
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returns it on the rest of the calls.
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.. literalinclude:: ../../examples/providers/singleton.py
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:language: python
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:lines: 3-
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``Singleton`` provider handles an injection of the dependencies the same way like a
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:ref:`factory-provider`.
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.. note::
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``Singleton`` provider does dependencies injection only when creates the object. When the object
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is created and memorized ``Singleton`` provider just returns it without applying the injections.
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Specialization of the provided type and abstract singletons work the same like like for the
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factories:
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- :ref:`factory-specialize-provided-type`
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- :ref:`abstract-factory`
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``Singleton`` provider scope is tied to the container. Two different containers will provider
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two different singleton objects:
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.. literalinclude:: ../../examples/providers/singleton_multiple_containers.py
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:language: python
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:lines: 3-
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Resetting memorized object
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--------------------------
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To reset a memorized object you need to call the ``.reset()`` method of the ``Singleton``
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provider.
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.. literalinclude:: ../../examples/providers/singleton_resetting.py
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:language: python
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:lines: 3-
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:emphasize-lines: 18
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.. note::
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Resetting of the memorized object clears the reference to it. Further object's lifecycle is
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managed by the garbage collector.
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Using singleton with multiple threads
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-------------------------------------
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``Singleton`` provider is NOT thread-safe. You need to explicitly establish a synchronization for
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using the ``Singleton`` provider in the multi-threading application. Otherwise you could trap
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into the race condition problem: ``Singleton`` will create multiple objects.
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There are two thread-safe singleton implementations out of the box:
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+ :py:class:`ThreadSafeSingleton` - is a thread-safe version of a ``Singleton`` provider. You can use
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in multi-threading applications without additional synchronization.
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+ :py:class:`ThreadLocalSingleton` - is a singleton provider that uses thread-locals as a storage.
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This type of singleton will manage multiple objects - the one object for the one thread.
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.. literalinclude:: ../../examples/providers/singleton_thread_locals.py
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:language: python
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:lines: 3-
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:emphasize-lines: 13,15
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Implementing scopes
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-------------------
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To implement a scoped singleton provider use a ``Singleton`` provider and reset its scope when
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needed.
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.. literalinclude:: ../../examples/providers/singleton_scoped.py
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:language: python
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:lines: 3-
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The output should look like this (each request a ``Service`` object has a different address):
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.. code-block:: bash
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* Serving Flask app "singleton_scoped" (lazy loading)
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* Environment: production
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WARNING: This is a development server. Do not use it in a production deployment.
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Use a production WSGI server instead.
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* Debug mode: off
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* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
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<__main__.Service object at 0x1099a9d90>
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127.0.0.1 - - [25/Aug/2020 17:33:11] "GET / HTTP/1.1" 200 -
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<__main__.Service object at 0x1099a9cd0>
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127.0.0.1 - - [25/Aug/2020 17:33:17] "GET / HTTP/1.1" 200 -
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<__main__.Service object at 0x1099a9d00>
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127.0.0.1 - - [25/Aug/2020 17:33:18] "GET / HTTP/1.1" 200 -
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<__main__.Service object at 0x1099a9e50>
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127.0.0.1 - - [25/Aug/2020 17:33:18] "GET / HTTP/1.1" 200 -
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<__main__.Service object at 0x1099a9d90>
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127.0.0.1 - - [25/Aug/2020 17:33:18] "GET / HTTP/1.1" 200 -
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.. disqus::
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