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115 lines
4.0 KiB
ReStructuredText
115 lines
4.0 KiB
ReStructuredText
Singleton providers
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-------------------
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.. currentmodule:: dependency_injector.providers
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:py:class:`Singleton` provider creates new instance of specified class on
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first call and returns same instance on every next call.
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Example:
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.. image:: /images/providers/singleton.png
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:width: 80%
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:align: center
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.. literalinclude:: ../../examples/providers/singleton.py
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:language: python
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:linenos:
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Singleton providers resetting
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Created and memorized by :py:class:`Singleton` instance can be reset. Reset of
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:py:class:`Singleton`'s memorized instance is done by clearing reference to
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it. Further lifecycle of memorized instance is out of :py:class:`Singleton`
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provider's control and depends on garbage collection strategy.
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Example:
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.. literalinclude:: ../../examples/providers/singleton_resetting.py
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:language: python
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:linenos:
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Singleton providers and injections
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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:py:class:`Singleton` provider has same interface as :py:class:`Factory`
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provider, so, all of the rules about injections are the same, as for
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:py:class:`Factory` provider.
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.. note::
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Due that :py:class:`Singleton` provider creates specified class instance
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only on the first call, all injections are done once, during the first
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call. Every next call, while instance has been already created
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and memorized, no injections are done, :py:class:`Singleton` provider just
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returns memorized earlier instance.
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This may cause some problems, for example, in case of trying to bind
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:py:class:`Factory` provider with :py:class:`Singleton` provider (provided
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by dependent :py:class:`Factory` instance will be injected only once,
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during the first call). Be aware that such behaviour was made with opened
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eyes and is not a bug.
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By the way, in such case, :py:class:`Delegate` or
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:py:class:`DelegatedSingleton` provider can be useful
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. It makes possible to inject providers *as is*. Please check out
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`Singleton providers delegation`_ section.
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Singleton providers delegation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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:py:class:`Singleton` provider could be delegated to any other provider via
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any kind of injection.
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Delegation of :py:class:`Singleton` providers is the same as
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:py:class:`Factory` providers delegation, please follow
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:ref:`factory_providers_delegation` section for examples (with exception
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of using :py:class:`DelegatedSingleton` instead of
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:py:class:`DelegatedFactory`).
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Singleton providers specialization
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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:py:class:`Singleton` provider could be specialized for any kind of needs via
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declaring its subclasses.
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Specialization of :py:class:`Singleton` providers is the same as
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:py:class:`Factory` providers specialization, please follow
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:ref:`factory_providers_specialization` section for examples.
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Abstract singleton providers
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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:py:class:`AbstractSingleton` provider is a :py:class:`Singleton` provider that
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must be explicitly overridden before calling.
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Behaviour of :py:class:`AbstractSingleton` providers is the same as of
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:py:class:`AbstractFactory`, please follow :ref:`abstract_factory_providers`
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section for examples (with exception of using :py:class:`AbstractSingleton`
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provider instead of :py:class:`AbstractFactory`).
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Singleton providers and multi-threading
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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:py:class:`Singleton` provider is NOT thread-safe and should be used in
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multi-threading applications with manually controlled locking.
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:py:class:`ThreadSafeSingleton` is a thread-safe version of
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:py:class:`Singleton` and could be used in multi-threading applications
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without any additional locking.
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Also there could be a need to use thread-scoped singletons and there is a
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special provider for such case - :py:class:`ThreadLocalSingleton`.
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:py:class:`ThreadLocalSingleton` provider creates instance once for each
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thread and returns it on every call.
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Example:
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.. literalinclude:: ../../examples/providers/singleton_thread_locals.py
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:language: python
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:linenos:
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.. disqus::
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