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243 lines
6.8 KiB
ReStructuredText
243 lines
6.8 KiB
ReStructuredText
Providers
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=========
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Providers are strategies of accessing objects.
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All providers are callable. They describe how particular objects will be
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provided.
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Instance providers & Injections
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-------------------------------
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*Instance* providers are providers that deal with object's creation and
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initialization.
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There are few *Instance* providers:
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- ``NewInstance`` provider creates new instance of specified class on every
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call.
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- ``Singleton`` provider creates new instance of specified class on first
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call and returns same instance on every next call.
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.. code-block:: python
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"""`NewInstance` and `Singleton` providers example."""
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from objects.providers import NewInstance
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from objects.providers import Singleton
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# NewInstance provider creates new instance of specified class on every call.
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new_object = NewInstance(object)
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object_1 = new_object()
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object_2 = new_object()
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assert object_1 is not object_2
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assert isinstance(object_1, object) and isinstance(object_2, object)
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# Singleton provider creates new instance of specified class on first call
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# and returns same instance on every next call.
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single_object = Singleton(object)
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single_object_1 = single_object()
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single_object_2 = single_object()
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assert single_object_1 is single_object_2
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assert isinstance(object_1, object) and isinstance(object_2, object)
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Objects can take dependencies in various forms. Some objects take init
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arguments, other are using attributes or methods to be initialized. It affects
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how such objects need to be created and initialized, and that is the place
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where **Injections** need to be used.
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In terms of computer science, **Injection** of dependency is a way how
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dependency can be coupled with dependent object.
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In terms of **Objects**, **Injection** is an instruction how to provide
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dependency for the particular object.
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Every Python object could be an injection's value. Special case is an **Objects**
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provider as an injection's value. In such case, injection value is a result of
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injectable provider call (every time injection is done).
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There are several types of injections. Below is a description of how they are
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used by instance providers:
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- ``KwArg`` - is injected in object's ``__init__()`` method in time of
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object's initialization via keyword argument.
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- ``Attribute`` - is injected into object's attribute (not class attribute)
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after object's initialization.
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- ``Method`` - is injected into object method's call after objects
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initialization.
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Example:
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.. code-block:: python
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"""`NewInstance` and `Singleton` providers with injections example."""
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import sqlite3
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from objects.providers import Singleton
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from objects.providers import NewInstance
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from objects.injections import KwArg
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from objects.injections import Attribute
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class ObjectA(object):
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"""ObjectA has dependency on database."""
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def __init__(self, database):
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"""Initializer.
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Database dependency need to be injected via init arg."""
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self.database = database
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def get_one(self):
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"""Select one from database and return it."""
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return self.database.execute('SELECT 1').fetchone()[0]
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# Database and `ObjectA` providers.
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database = Singleton(sqlite3.Connection,
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KwArg('database', ':memory:'),
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KwArg('timeout', 30),
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KwArg('detect_types', True),
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KwArg('isolation_level', 'EXCLUSIVE'),
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Attribute('row_factory', sqlite3.Row))
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object_a = NewInstance(ObjectA,
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KwArg('database', database))
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# Creating several `ObjectA` instances.
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object_a_1 = object_a()
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object_a_2 = object_a()
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# Making some asserts.
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assert object_a_1 is not object_a_2
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assert object_a_1.database is object_a_2.database is database()
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assert object_a_1.get_one() == object_a_2.get_one() == 1
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Static providers
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----------------
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Static providers are family of providers that returns its values "as is".
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There are four of static providers: ``Class``, ``Object``, ``Function`` and
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``Value``. All of them has the same behaviour, but usage of anyone is
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predicted by readability and providable object's type.
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.. code-block:: python
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"""Static providers example."""
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from objects.providers import Class
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from objects.providers import Object
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from objects.providers import Function
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from objects.providers import Value
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cls_provider = Class(object)
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assert cls_provider() is object
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object_provider = Object(object())
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assert isinstance(object_provider(), object)
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function_provider = Function(len)
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assert function_provider() is len
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value_provider = Value(123)
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assert value_provider() == 123
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Callable provider
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-----------------
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External dependency provider
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----------------------------
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Config provider
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---------------
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Providers delegation
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--------------------
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Overriding of providers
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-----------------------
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Any provider can be overridden by another provider.
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Example:
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.. code-block:: python
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"""Provider overriding example."""
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import sqlite3
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from objects.providers import Singleton
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from objects.providers import NewInstance
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from objects.injections import KwArg
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from objects.injections import Attribute
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class ObjectA(object):
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"""ObjectA has dependency on database."""
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def __init__(self, database):
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"""Initializer.
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Database dependency need to be injected via init arg."""
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self.database = database
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def get_one(self):
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"""Select one from database and return it."""
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return self.database.execute('SELECT 1')
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class ObjectAMock(ObjectA):
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"""Mock of ObjectA.
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Has no dependency on database.
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"""
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def __init__(self):
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"""Initializer."""
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def get_one(self):
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"""Select one from database and return it.
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Mock makes no database queries and always returns two instead of one.
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"""
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return 2
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# Database and `ObjectA` providers.
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database = Singleton(sqlite3.Connection,
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KwArg('database', ':memory:'),
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KwArg('timeout', 30),
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KwArg('detect_types', True),
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KwArg('isolation_level', 'EXCLUSIVE'),
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Attribute('row_factory', sqlite3.Row))
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object_a = NewInstance(ObjectA,
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KwArg('database', database))
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# Overriding `ObjectA` provider with `ObjectAMock` provider.
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object_a.override(NewInstance(ObjectAMock))
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# Creating several `ObjectA` instances.
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object_a_1 = object_a()
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object_a_2 = object_a()
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# Making some asserts.
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assert object_a_1 is not object_a_2
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assert object_a_1.get_one() == object_a_2.get_one() == 2
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