Catalogs
========

Catalogs are collections of providers. Main purpose of catalogs is to group
providers. 

There are, actually, several popular cases of catalogs usage:

- Grouping of providers from the same architectural layer (for example, 
  ``Services``, ``Models`` and ``Forms`` catalogs).
- Grouping of providers from the same functional groups (for example,
  catalog ``Users``, that contains all functional parts of ``Users``
  component).

Also, for both of these and some other cases, it might be useful to attach 
some init / shutdown functionality or something else, that deals with group 
of providers.

Writing catalogs
----------------

Catalogs have to extend base catalog class ``di.AbstractCatalog``.

Providers have to be defined like catalog's attributes. Every provider in
catalog has name. This name should follow ``some_provider`` convention, 
that is standard naming convention for attribute names in Python.

.. note::

    It might be useful to add such ``""":type: (di.Provider) -> Object1"""`` 
    documentation blocks one line after provider definition for every provider.
    It will help code analyzing tools and IDE's to understand that variable 
    above contains some callable object, that returns particular instance 
    in a result of call.

Example:

.. image:: /images/catalogs/simple.png
    :width: 100%
    :align: center

.. literalinclude:: ../../examples/catalogs/simple.py
   :language: python

Operating with catalog providers
--------------------------------

``di.AbstractCatalog`` has several features that could be useful for some kind 
of operations on catalog's providers:

- ``di.AbstractCatalog.providers`` is read-only attribute that contains 
  ``dict`` of all catalog providers, including providers that are inherited 
  from parent catalogs, where key is the name of provider and value is 
  provider itself.
- ``di.AbstractCatalog.cls_providers`` is read-only attribute contains ``dict``
  of current catalog providers, where key is the name of provider and value is 
  provider itself.
- ``di.AbstractCatalog.inherited_providers`` is read-only attribute contains 
  ``dict`` of all providers that are inherited from parent catalogs, where key 
  is the name of provider and value is provider itself. 
- ``di.AbstractCatalog.filter(provider_type=di.Provider)`` is a class method 
  that could be used for filtering catalog providers by provider types 
  (for example, for getting all ``di.Factory`` providers). 
  ``di.AbstractCatalog.filter()`` method use ``di.AbstractCatalog.providers``.

Example:

.. literalinclude:: ../../examples/catalogs/operating_with_providers.py
   :language: python

Overriding of catalogs
----------------------

Catalogs can be overridden by other catalogs. This, actually, means that 
all of the providers from overriding catalog will override providers with the 
same names in overridden catalog.

There are two ways to override catalog by another catalog:

- Use ``di.AbstractCatalog.override(AnotherCatalog)`` method.
- Use ``@di.override(AnotherCatalog)`` class decorator.

Example of overriding catalog using ``di.AbstractCatalog.override()`` method:

.. literalinclude:: ../../examples/catalogs/override.py
   :language: python

Example of overriding catalog using ``@di.override()`` decorator:

.. literalinclude:: ../../examples/catalogs/override_decorator.py
   :language: python