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Adding Singleton docs & making some changes for Factory docs

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Roman Mogilatov 2015-06-10 12:00:43 +03:00
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docs/_providers.rst
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Providers
=========
Providers are strategies of accessing objects.
All providers are callable. They describe how particular objects are provided.
Factory providers
-----------------
``Factory`` provider creates new instance of specified class on every call.
Nothing could be better than brief example:
.. code-block:: python
"""`Factory` providers example."""
from objects.providers import Factory
class User(object):
"""Example class User."""
# Factory provider creates new instance of specified class on every call.
users_factory = Factory(User)
user1 = users_factory()
user2 = users_factory()
assert user1 is not user2
assert isinstance(user1, User) and isinstance(user2, User)
Factory providers and injections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Objects can take dependencies in different forms. Some objects take init
arguments, other are using attributes setting or method calls to be
initialized. It affects how such objects need to be created and initialized,
and that is the place where ``objects.injections`` need to be used.
``Factory`` provider takes various number of positional arguments, that define
what kind of dependency injections need to be done.
All of those instructions are defined in ``objects.injections`` module and are
subclasses of ``objects.injections.Injection``. There are several types of
injections that are used by ``Factory`` provider:
- ``KwArg`` - injection is done by passing injectable value in object's
``__init__()`` method in time of object's creation via keyword argument.
Takes keyword name of ``__init__()`` argument and injectable value.
- ``Attribute`` - injection is done by setting specified attribute with
injectable value right after object's creation. Takes attribute name and
injectable value.
- ``Method`` - injection is done by calling of specified method with
injectable value right after object's creation and attribute injections
are done. Takes method name and injectable value.
All ``Injection``'s injectable values are provided *"as is"*, except of
providers. Providers will be called every time, when injection needs to be
done.
Factory providers and __init__ injections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example below shows how to create ``Factory`` of particular class with
``__init__`` keyword argument injections which injectable values are also
provided by another factories:
.. image:: ../images/factory_init_injections.png
.. code-block:: python
"""`Factory` providers with init injections example."""
from objects.providers import Factory
from objects.injections import KwArg
class User(object):
"""Example class User."""
def __init__(self, main_photo):
"""Initializer.
:param main_photo: Photo
:return:
"""
self.main_photo = main_photo
super(User, self).__init__()
class Photo(object):
"""Example class Photo."""
# User and Photo factories:
photos_factory = Factory(Photo)
users_factory = Factory(User,
KwArg('main_photo', photos_factory))
# Creating several User objects:
user1 = users_factory() # Same as: user1 = User(main_photo=Photo())
user2 = users_factory() # Same as: user2 = User(main_photo=Photo())
# Making some asserts:
assert isinstance(user1, User)
assert isinstance(user1.main_photo, Photo)
assert isinstance(user2, User)
assert isinstance(user2.main_photo, Photo)
assert user1 is not user2
assert user1.main_photo is not user2.main_photo
Next example shows how ``Factory`` provider deals with positional and keyword
``__init__`` context arguments. In few words, ``Factory`` provider fully
passes positional context arguments to class's ``__init__`` method, but
keyword context arguments have priority on ``KwArg`` injections (this could be
useful for testing). So, please, follow the example below:
.. image:: ../images/factory_init_injections_and_contexts.png
.. code-block:: python
"""`Factory` providers with init injections and context arguments example."""
from objects.providers import Factory
from objects.injections import KwArg
class User(object):
"""Example class User.
Class User has to be provided with user id.
Also Class User has dependencies on class Photo and class CreditCard
objects.
All of the dependencies have to be provided like __init__ arguments.
"""
def __init__(self, id, main_photo, credit_card):
"""Initializer.
:param id: int
:param main_photo: Photo
:param credit_card: CreditCard
:return:
"""
self.id = id
self.main_photo = main_photo
self.credit_card = credit_card
super(User, self).__init__()
class Photo(object):
"""Example class Photo."""
class CreditCard(object):
"""Example class CreditCard."""
# User, Photo and CreditCard factories:
credit_cards_factory = Factory(CreditCard)
photos_factory = Factory(Photo)
users_factory = Factory(User,
KwArg('main_photo', photos_factory),
KwArg('credit_card', credit_cards_factory))
# Creating several User objects:
user1 = users_factory(1) # Same as: user1 = User(1,
# main_photo=Photo(),
# credit_card=CreditCard())
user2 = users_factory(2) # Same as: user2 = User(2,
# main_photo=Photo(),
# credit_card=CreditCard())
# Making some asserts:
assert user1.id == 1
assert isinstance(user1.main_photo, Photo)
assert isinstance(user1.credit_card, CreditCard)
assert user2.id == 2
assert isinstance(user2.main_photo, Photo)
assert isinstance(user2.credit_card, CreditCard)
assert user1.main_photo is not user2.main_photo
assert user1.credit_card is not user2.credit_card
# Context keyword arguments have priority on KwArg injections priority:
main_photo_mock = Photo()
credit_card_mock = CreditCard()
user3 = users_factory(3, main_photo=main_photo_mock,
credit_card=credit_card_mock)
assert user3.id == 3
assert user3.main_photo is main_photo_mock
assert user3.credit_card is credit_card_mock
Factory providers and attribute injections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example below shows how to create ``Factory`` of particular class with
attribute injections. Those injections are done by setting specified attributes
with injectable values right after object's creation.
.. image:: ../images/factory_attribute_injections.png
.. code-block:: python
"""`Factory` providers with attribute injections example."""
from objects.providers import Factory
from objects.injections import Attribute
class User(object):
"""Example class User."""
def __init__(self):
"""Initializer."""
self.main_photo = None
self.credit_card = None
class Photo(object):
"""Example class Photo."""
class CreditCard(object):
"""Example class CreditCard."""
# User, Photo and CreditCard factories:
credit_cards_factory = Factory(CreditCard)
photos_factory = Factory(Photo)
users_factory = Factory(User,
Attribute('main_photo', photos_factory),
Attribute('credit_card', credit_cards_factory))
# Creating several User objects:
user1 = users_factory() # Same as: user1 = User()
# user1.main_photo = Photo()
# user1.credit_card = CreditCard()
user2 = users_factory() # Same as: user2 = User()
# user2.main_photo = Photo()
# user2.credit_card = CreditCard()
# Making some asserts:
assert user1 is not user2
assert isinstance(user1.main_photo, Photo)
assert isinstance(user1.credit_card, CreditCard)
assert isinstance(user2.main_photo, Photo)
assert isinstance(user2.credit_card, CreditCard)
assert user1.main_photo is not user2.main_photo
assert user1.credit_card is not user2.credit_card
Factory providers and method injections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Current example shows how to create ``Factory`` of particular class with
method injections. Those injections are done by calling of specified method
with injectable value right after object's creation and attribute injections
are done.
Method injections are not very popular in Python due Python best practices
(usage of public attributes instead of setter methods), but it may appear in
some cases.
.. image:: ../images/factory_method_injections.png
.. code-block:: python
"""`Factory` providers with method injections example."""
from objects.providers import Factory
from objects.injections import Method
class User(object):
"""Example class User."""
def __init__(self):
"""Initializer."""
self.main_photo = None
self.credit_card = None
def set_main_photo(self, photo):
"""Set user's main photo."""
self.main_photo = photo
def set_credit_card(self, credit_card):
"""Set user's credit card."""
self.credit_card = credit_card
class Photo(object):
"""Example class Photo."""
class CreditCard(object):
"""Example class CreditCard."""
# User, Photo and CreditCard factories:
credit_cards_factory = Factory(CreditCard)
photos_factory = Factory(Photo)
users_factory = Factory(User,
Method('set_main_photo', photos_factory),
Method('set_credit_card', credit_cards_factory))
# Creating several User objects:
user1 = users_factory() # Same as: user1 = User()
# user1.set_main_photo(Photo())
# user1.set_credit_card(CreditCard())
user2 = users_factory() # Same as: user2 = User()
# user2.set_main_photo(Photo())
# user2.set_credit_card(CreditCard())
# Making some asserts:
assert user1 is not user2
assert isinstance(user1.main_photo, Photo)
assert isinstance(user1.credit_card, CreditCard)
assert isinstance(user2.main_photo, Photo)
assert isinstance(user2.credit_card, CreditCard)
assert user1.main_photo is not user2.main_photo
assert user1.credit_card is not user2.credit_card
Singleton providers
-------------------
``Singleton`` provider creates new instance of specified class on first call
and returns same instance on every next call.
Instance providers & Injections
-------------------------------
Providers
~~~~~~~~~
*Instance* providers are providers that deal with object's creation and
initialization.
There are few *Instance* providers:
- ``Factory`` provider creates new instance of specified class on every
call.
- ``Singleton`` provider creates new instance of specified class on first
call and returns same instance on every next call.
Example:
.. code-block:: python
"""`Factory` and `Singleton` providers example."""
from objects.providers import Factory
from objects.providers import Singleton
# Factory provider creates new instance of specified class on every call.
object_factory = Factory(object)
object_1 = object_factory()
object_2 = object_factory()
assert object_1 is not object_2
assert isinstance(object_1, object) and isinstance(object_2, object)
# Singleton provider creates new instance of specified class on first call
# and returns same instance on every next call.
single_object = Singleton(object)
single_object_1 = single_object()
single_object_2 = single_object()
assert single_object_1 is single_object_2
assert isinstance(object_1, object) and isinstance(object_2, object)
Injections
~~~~~~~~~~
Objects can take dependencies in various forms. Some objects take init
arguments, other are using attributes or methods to be initialized. It affects
how such objects need to be created and initialized, and that is the place
where *Injections* need to be used.
In terms of computer science, *Injection of dependency* is a way how
dependency can be coupled with dependent object.
In terms of *Objects*, *Injection* is an instruction how to provide
dependency for the particular provider.
Every Python object could be an injection's value. Special case is an *Objects*
provider as an injection's value. In such case, injection value is a result of
injectable provider call (every time injection is done).
There are several types of injections. Below is a description of how they are
used by instance providers:
- ``KwArg`` - is injected in object's ``__init__()`` method in time of
object's initialization via keyword argument.
- ``Attribute`` - is injected into object's attribute (not class attribute)
after object's initialization.
- ``Method`` - is injected into object method's call after objects
initialization.
Example:
.. code-block:: python
"""`Factory` and `Singleton` providers with injections example."""
import sqlite3
from objects.providers import Singleton
from objects.providers import Factory
from objects.injections import KwArg
from objects.injections import Attribute
class ObjectA(object):
"""ObjectA has dependency on database."""
def __init__(self, database):
"""Initializer.
Database dependency need to be injected via init arg."""
self.database = database
def get_one(self):
"""Select one from database and return it."""
return self.database.execute('SELECT 1').fetchone()[0]
# Database and `ObjectA` providers.
database = Singleton(sqlite3.Connection,
KwArg('database', ':memory:'),
KwArg('timeout', 30),
KwArg('detect_types', True),
KwArg('isolation_level', 'EXCLUSIVE'),
Attribute('row_factory', sqlite3.Row))
object_a_factory = Factory(ObjectA,
KwArg('database', database))
# Creating several `ObjectA` instances.
object_a_1 = object_a_factory()
object_a_2 = object_a_factory()
# Making some asserts.
assert object_a_1 is not object_a_2
assert object_a_1.database is object_a_2.database is database()
assert object_a_1.get_one() == object_a_2.get_one() == 1
Static providers
----------------

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# Factory provider creates new instance of specified class on every call.
users_factory = Factory(User)
# Creating several User objects:
user1 = users_factory()
user2 = users_factory()
# Making some asserts:
assert user1 is not user2
assert isinstance(user1, User) and isinstance(user2, User)

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Singleton providers
-------------------
``Singleton`` provider creates new instance of specified class on first call
and returns same instance on every next call.
.. code-block:: python
"""`Singleton` providers example."""
from objects.providers import Singleton
class UserService(object):
"""Example class UserService."""
# Singleton provider creates new instance of specified class on first call and
# returns same instance on every next call.
users_service_provider = Singleton(UserService)
# Retrieving several UserService objects:
user_service1 = users_service_provider()
user_service2 = users_service_provider()
# Making some asserts:
assert user_service1 is user_service2
assert isinstance(user_service1, UserService)
assert isinstance(user_service2, UserService)
Singleton providers and injections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
``Singleton`` providers use ``Factory`` providers for first creation of
specified class instance, so, all of the rules about injections are the same,
as for ``Factory`` providers.
.. note::
Due that ``Singleton`` provider creates specified class instance only on
the first call, all injections are done once, during the first call, also.
Every next call, while instance has been already created and memorized, no
injections are done, ``Singleton`` provider just returns memorized earlier
instance.
This may cause some problems, for example, in case of trying to bind
``Factory`` provider with ``Singleton`` provider (provided by dependent
``Factory`` instance will be injected only once, during the first call).
Be aware that such behaviour was made with opened eyes and is not a bug.
By the way, in such case, ``Delegate`` provider can be useful. It makes
possible to inject providers *as is*. Please check out full example in
*Providers delegation* section.
Singleton providers resetting
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Created and memorized by ``Singleton`` instance can be reset. Reset of
``Singleton``'s memorized instance is done by clearing reference to it. Further
lifecycle of memorized instance is out of ``Singleton`` provider's control.
.. code-block:: python
"""`Singleton` providers resetting example."""
from objects.providers import Singleton
class UserService(object):
"""Example class UserService."""
# Singleton provider creates new instance of specified class on first call and
# returns same instance on every next call.
users_service_provider = Singleton(UserService)
# Retrieving several UserService objects:
user_service1 = users_service_provider()
user_service2 = users_service_provider()
# Making some asserts:
assert user_service1 is user_service2
assert isinstance(user_service1, UserService)
assert isinstance(user_service2, UserService)
# Resetting of memorized instance:
users_service_provider.reset()
# Retrieving one more UserService object:
user_service3 = users_service_provider()
# Making some asserts:
assert user_service3 is not user_service1

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examples/providers/factory.py
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@ -11,8 +11,10 @@ class User(object):
# Factory provider creates new instance of specified class on every call.
users_factory = Factory(User)
# Creating several User objects:
user1 = users_factory()
user2 = users_factory()
# Making some asserts:
assert user1 is not user2
assert isinstance(user1, User) and isinstance(user2, User)

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"""`Singleton` providers example."""
from objects.providers import Singleton
class UserService(object):
"""Example class UserService."""
# Singleton provider creates new instance of specified class on first call and
# returns same instance on every next call.
users_service_provider = Singleton(UserService)
# Retrieving several UserService objects:
user_service1 = users_service_provider()
user_service2 = users_service_provider()
# Making some asserts:
assert user_service1 is user_service2
assert isinstance(user_service1, UserService)
assert isinstance(user_service2, UserService)

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"""`Singleton` providers resetting example."""
from objects.providers import Singleton
class UserService(object):
"""Example class UserService."""
# Singleton provider creates new instance of specified class on first call and
# returns same instance on every next call.
users_service_provider = Singleton(UserService)
# Retrieving several UserService objects:
user_service1 = users_service_provider()
user_service2 = users_service_provider()
# Making some asserts:
assert user_service1 is user_service2
assert isinstance(user_service1, UserService)
assert isinstance(user_service2, UserService)
# Resetting of memorized instance:
users_service_provider.reset()
# Retrieving one more UserService object:
user_service3 = users_service_provider()
# Making some asserts:
assert user_service3 is not user_service1