Providers ========= Providers are strategies of accessing objects. All providers are callable. They describe how particular objects will be provided. Instance providers & Injections ------------------------------- *Instance* providers are providers that deal with object's creation and initialization. There are few *Instance* providers: - ``NewInstance`` 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 """`NewInstance` and `Singleton` providers example.""" from objects.providers import NewInstance from objects.providers import Singleton # NewInstance provider creates 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 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) 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 object. 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 """`NewInstance` and `Singleton` providers with 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 is database() assert object_a_1.get_one() == object_a_2.get_one() == 1 Static providers ---------------- Static providers are family of providers that return their values "as is". There are four of static providers: ``Class``, ``Object``, ``Function`` and ``Value``. All of them has the same behaviour, but usage of anyone is predicted by readability and providable object's type. Example: .. code-block:: python """Static providers example.""" from objects.providers import Class from objects.providers import Object from objects.providers import Function from objects.providers import Value cls_provider = Class(object) assert cls_provider() is object object_provider = Object(object()) assert isinstance(object_provider(), object) function_provider = Function(len) assert function_provider() is len value_provider = Value(123) assert value_provider() == 123 Callable providers ------------------ ``Callable`` provider is a provider that decorates particular callable with some injections. Every call of this provider returns result of call of initial callable. Example: .. code-block:: python """`Callable` providers examples.""" from objects.providers import Callable from objects.providers import Singleton from objects.injections import KwArg import sqlite3 def some_function(arg, database): """Example function that has input arg and dependency on database.""" return database.execute('SELECT @1', [arg]).fetchone()[0] # Database and `ObjectA` providers. database = Singleton(sqlite3.Connection, KwArg('database', ':memory:')) some_function = Callable(some_function, KwArg('database', database)) # Some asserts. assert some_function(1) == 1 assert some_function(2) == 2 assert some_function(2231) == 2231 External dependency providers ----------------------------- ``ExternalDependency`` provider can be useful for development of self-sufficient libraries / modules / applications, that has required external dependencies. For example, you have created self-sufficient library / module / application, that has dependency on *SQLAlchemy*. Second step you want to do is to make this software component to be easy reusable by wide amount of developers and to be easily integrated into many applications. It may be good idea, to move all external dependencies (like *SQLAlchemy*) to the top level and make them to be injected on your software component's initialization. It will make third party developers feel themselves more free about integration of yours component in their applications because of they would be able to find right place / right way for doing this in their application's architectures. Example: .. code-block:: python import this Config providers ---------------- Providers delegation -------------------- Overriding of providers ----------------------- Any provider can be overridden by another provider. Example: .. code-block:: python """Providers overriding 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') class ObjectAMock(ObjectA): """Mock of ObjectA. Has no dependency on database. """ def __init__(self): """Initializer.""" def get_one(self): """Select one from database and return it. Mock makes no database queries and always returns two instead of one. """ return 2 # 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)) # Overriding `ObjectA` provider with `ObjectAMock` provider. object_a.override(NewInstance(ObjectAMock)) # 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.get_one() == object_a_2.get_one() == 2