python-dependency-injector/docs/entities.rst

Entities
========

Current section describes main `Objects` entities and their interaction.

Providers
---------

Providers are strategies of accessing objects.

All providers are callable. They describe how particular objects will be
provided. For example:

.. code-block:: python

    """`NewInstance` and `Singleton` providers example."""

    from objects.providers import NewInstance
    from objects.providers import Singleton


    # NewInstance provider will create new instance of specified class
    # on every call.
    new_object = NewInstance(object)

    object_1 = new_object()
    object_2 = new_object()

    assert object_1 is not object_2

    # Singleton provider will create new instance of specified class on first call,
    # and return 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

Injections
----------

Injections are additional instructions, that are used for determining
dependencies of objects.

Objects can take dependencies in various forms. Some objects take init
arguments, other are using attributes or methods to be initialized. Injection,
in terms of `Objects`, is an instruction how to provide dependency for the
particular object.

Every Python object could be an injection's value. Special case is a `Objects`
provider as an injection's value. In such case, injection value is a result of
injectable provider call (every time injection is done).

Injections are used by providers.

.. code-block:: python

    """`KwArg` and `Attribute` injections example."""

    import sqlite3

    from objects.providers import Singleton
    from objects.providers import NewInstance

    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 = NewInstance(ObjectA,
                           KwArg('database', database))

    # Creating several `ObjectA` instances.
    object_a_1 = object_a()
    object_a_2 = object_a()

    # Making some asserts.
    assert object_a_1 is not object_a_2
    assert object_a_1.database is object_a_2.database
    assert object_a_1.get_one() == object_a_2.get_one() == 1

Catalogs
--------

Catalogs are named set of providers.

`Objects` catalogs can be used for grouping of providers by some
kind of rules. In example below, there are two catalogs:
`Resources` and `Models`.

`Resources` catalog is used to group all common application resources like
database connection and various api clients, while `Models` catalog is used
for application model providers only.

.. code-block:: python

    """Catalogs example."""

    import sqlite3
    import httplib

    from objects.catalog import AbstractCatalog

    from objects.providers import Singleton
    from objects.providers import NewInstance

    from objects.injections import KwArg
    from objects.injections import Attribute


    class SomeModel(object):

        """SomeModel has dependency on database and api client.

        Dependencies need to be injected via init args.
        """

        def __init__(self, database, api_client):
            """Initializer."""
            self.database = database
            self.api_client = api_client

        def api_request(self):
            """Make api request."""
            self.api_client.request('GET', '/')
            return self.api_client.getresponse()

        def get_one(self):
            """Select one from database and return it."""
            return self.database.execute('SELECT 1').fetchone()[0]


    class Resources(AbstractCatalog):

        """Resource providers catalog."""

        database = Singleton(sqlite3.Connection,
                             KwArg('database', ':memory:'),
                             KwArg('timeout', 30),
                             KwArg('detect_types', True),
                             KwArg('isolation_level', 'EXCLUSIVE'),
                             Attribute('row_factory', sqlite3.Row))

        api_client = Singleton(httplib.HTTPConnection,
                               KwArg('host', 'example.com'),
                               KwArg('port', 80),
                               KwArg('timeout', 10))


    class Models(AbstractCatalog):

        """Model providers catalog."""

        some_model = NewInstance(SomeModel,
                                 KwArg('database', Resources.database),
                                 KwArg('api_client', Resources.api_client))


    # Creating `SomeModel` instance.
    some_model = Models.some_model()

    # Making some asserts.
    assert some_model.get_one() == 1
    assert some_model.api_request().status == 200