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"""
Utility for having class declarations automatically cause a reference to
the class to be stored along with its name.
This pattern enables the associated class to be looked up later by using
its name.
"""
from __future__ import absolute_import
# Specifying 'LEAVE_UNREGISTERED' as the "REGISTERED_NAME" attribute will cause the class to be
# omitted from the registry. This is particularly useful for base classes that define an interface
# or common functionality, and aren't intended to be constructed explicitly.
LEAVE_UNREGISTERED = object()
def make_registry_metaclass(registry_store):
"""
Returns a new Registry metaclass.
"""
if not isinstance(registry_store, dict):
raise TypeError("'registry_store' argument must be a dict")
class Registry(type):
"""
A metaclass that stores a reference to all registered classes.
"""
def __new__(meta, class_name, base_classes, class_dict):
"""
Creates and returns a new instance of Registry, which is a
class named 'class_name' derived from 'base_classes' that
defines 'class_dict' as additional attributes.
The returned class is added to 'registry_store' using
class_dict["REGISTERED_NAME"] as the name, or 'class_name'
if the "REGISTERED_NAME" attribute isn't defined. If the
sentinel value 'LEAVE_UNREGISTERED' is specified as the
name, then the returned class isn't added to
'registry_store'.
The returned class will have the "REGISTERED_NAME" attribute
defined either as its associated key in 'registry_store' or
the 'LEAVE_UNREGISTERED' sentinel value.
"""
registered_name = class_dict.setdefault("REGISTERED_NAME", class_name)
cls = type.__new__(meta, class_name, base_classes, class_dict)
if registered_name is not LEAVE_UNREGISTERED:
if registered_name in registry_store:
raise ValueError("The name %s is already registered; a different value for the"
" 'REGISTERED_NAME' attribute must be chosen" %
(registered_name))
registry_store[registered_name] = cls
return cls
return Registry
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