Advanced Topics
This section covers more advanced or esoteric uses of ClassRegistry features.
Registering Classes Automatically
Tired of having to add the register
decorator to every class that you want
to add to a class registry? Surely there’s a better way!
ClassRegistry also provides an AutoRegister()
metaclass that you can
apply to a base class. Any non-abstract subclass that extends that base class
will be registered automatically.
Here’s an example:
from abc import abstractmethod
from class_registry import AutoRegister, ClassRegistry
pokedex = ClassRegistry('element')
# Note ``AutoRegister(pokedex)`` used as the metaclass here.
class Pokemon(metaclass=AutoRegister(pokedex)):
@abstractmethod
def get_abilities(self):
raise NotImplementedError()
# Define some non-abstract subclasses.
class Butterfree(Pokemon):
element = 'bug'
def get_abilities(self):
return ['compound_eyes']
class Spearow(Pokemon):
element = 'flying'
def get_abilities(self):
return ['keen_eye']
# Any non-abstract class that extends ``Pokemon`` will automatically
# get registered in our Pokédex!
assert list(pokedex.items()) == \
[('bug', Butterfree), ('flying', Spearow)]
In the above example, note that Butterfree
and Spearow
were added to
pokedex
automatically. However, the Pokemon
base class was not added,
because it is abstract.
Important
Python defines an abstract class as a class with at least one unimplemented
abstract method. You can’t just add metaclass=ABCMeta
!
from abc import ABCMeta
# Declare an "abstract" class.
class ElectricPokemon(Pokemon, metaclass=ABCMeta):
element = 'electric'
def get_abilities(self):
return ['shock']
assert list(pokedex.items()) == \
[('bug', Butterfree), \
('flying', Spearow), \
('electric', ElectricPokemon)]
Note in the above example that ElectricPokemon
was added to pokedex
,
even though its metaclass is ABCMeta
.
Because ElectricPokemon
doesn’t have any unimplemented abstract methods,
Python does not consider it to be abstract.
We can verify this by using inspect.isabstract()
:
from inspect import isabstract
assert not isabstract(ElectricPokemon)
Patching
From time to time, you might need to register classes temporarily. For example, you might need to patch a global class registry in a unit test, ensuring that the extra classes are removed when the test finishes.
ClassRegistry provides a RegistryPatcher
that you can use for just
such a purpose:
from class_registry import ClassRegistry, RegistryKeyError, \
RegistryPatcher
pokedex = ClassRegistry('element')
# Create a couple of new classes, but don't register them yet!
class Oddish(object):
element = 'grass'
class Meowth(object):
element = 'normal'
# As expected, neither of these classes are registered.
try:
pokedex['grass']
except RegistryKeyError:
pass
# Use a patcher to temporarily register these classes.
with RegistryPatcher(pokedex, Oddish, Meowth):
abbot = pokedex['grass']
assert isinstance(abbot, Oddish)
costello = pokedex['normal']
assert isinstance(costello, Meowth)
# Outside the context, the classes are no longer registered!
try:
pokedex['grass']
except RegistryKeyError:
pass
If desired, you can also change existing registry keys, or even replace a class that is already registered.
@pokedex.register
class Squirtle(object):
element = 'water'
# Get your diving suit Meowth; we're going to Atlantis!
with RegistryPatcher(pokedex, water=Meowth):
nemo = pokedex['water']
assert isinstance(nemo, Meowth)
# After the context exits, the previously-registered class is
# restored.
ponsonby = pokedex['water']
assert isinstance(ponsonby, Squirtle)
Important
Only mutable registries can be patched (any class that extends
BaseMutableRegistry
).
In particular, this means that EntryPointClassRegistry
can
not be patched using RegistryPatcher
.
Overriding Lookup Keys
In some cases, you may want to customise the way a ClassRegistry
looks up
which class to use. For example, you may need to change the registry key for a
particular class, but you want to maintain backwards-compatibility for existing
code that references the old key.
To customise this, create a subclass of ClassRegistry
and override its
gen_lookup_key
method:
class FacadeRegistry(ClassRegistry):
@staticmethod
def gen_lookup_key(key: str) -> str:
"""
In a previous version of the codebase, some pokémon had the 'bird'
type, but this was later dropped in favour of 'flying'.
"""
if key == 'bird':
return 'flying'
return key
pokedex = FacadeRegistry('element')
@pokedex.register
class MissingNo(Pokemon):
element = 'flying'
@pokedex.register
class Meowth(object):
element = 'normal'
# MissingNo can be accessed by either key.
assert isinstance(pokedex['bird'], MissingNo)
assert isinstance(pokedex['flying'], MissingNo)
# Other pokémon work as you'd expect.
assert isinstance(pokedex['normal'], Meowth)