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Factories vs. Registries
========================
Despite its name, :py:class:`ClassRegistry` also has aspects in common with the
Factory pattern.
Most notably, accessing a registry key automatically creates a new instance of
the corresponding class.
But, what if you want a :py:class:`ClassRegistry` to behave more strictly like a
registry — always returning the the `same` instance each time the same key is
accessed?
This is where :py:class:`ClassRegistryInstanceCache` comes into play. It wraps
a :py:class:`ClassRegistry` and provides a caching mechanism, so that each time
you access a particular key, it always returns the same instance for that key.
Let's see what this looks like in action:
.. code-block:: python
from class_registry import ClassRegistry, ClassRegistryInstanceCache
pokedex = ClassRegistry('element')
@pokedex.register
class Pikachu(object):
element = 'electric'
@pokedex.register
class Alakazam(object):
element = 'psychic'
fighters = ClassRegistryInstanceCache(pokedex)
# Accessing the ClassRegistry yields a different instance every
# time.
pika_1 = pokedex['electric']
assert isinstance(pika_1, Pikachu)
pika_2 = pokedex['electric']
assert isinstance(pika_2, Pikachu)
assert pika_1 is not pika_2
# ClassRegistryInstanceCache works just like ClassRegistry, except
# it returns the same instance per key.
pika_3 = fighters['electric']
assert isinstance(pika_3, Pikachu)
pika_4 = fighters['electric']
assert isinstance(pika_4, Pikachu)
assert pika_3 is pika_4
darth_vader = fighters['psychic']
assert isinstance(darth_vader, Alakazam)
anakin_skywalker = fighters['psychic']
assert isinstance(anakin_skywalker, Alakazam)
assert darth_vader is anakin_skywalker
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