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84 lines
2.8 KiB
ReStructuredText
84 lines
2.8 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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Singleton providers and injections
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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:py:class:`Singleton` provider extends :py:class:`Factory` provider, so, all
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of the rules about injections are the same, as for :py:class:`Factory`
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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, also. 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` provider can be useful. It
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makes possible to inject providers *as is*. Please check out full example
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in *Providers delegation* section.
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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.
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Example:
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.. literalinclude:: ../../examples/providers/singleton_reseting.py
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:language: python
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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. Delegation of :py:class:`Singleton` providers is the
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same as :py:class:`Factory` providers delegation, please follow *Factory
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providers delegation* section for example.
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:py:class:`Singleton` delegate could be created obviously using
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``Delegate(Singleton(...))`` or by calling ``Singleton(...).delegate()``
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method.
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Example:
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.. literalinclude:: ../../examples/providers/singleton_delegation.py
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:language: python
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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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One of such `builtin` features is a limitation to :py:class:`Singleton`
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provided type:
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.. literalinclude:: ../../examples/providers/singleton_provided_type.py
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:language: python
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