b8f6239094
Also defines a generic Registry metaclass for making class declarations store a reference to the class along with a name so that they can be looked up later.
62 lines
2.3 KiB
Python
62 lines
2.3 KiB
Python
"""
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Utility for having class declarations automatically cause a reference to
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the class to be stored along with its name.
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This pattern enables the associated class to be looked up later by using
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its name.
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"""
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from __future__ import absolute_import
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# Specifying 'LEAVE_UNREGISTERED' as the "REGISTERED_NAME" attribute will cause the class to be
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# omitted from the registry. This is particularly useful for base classes that define an interface
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# or common functionality, and aren't intended to be constructed explicitly.
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LEAVE_UNREGISTERED = object()
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def make_registry_metaclass(registry_store):
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"""
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Returns a new Registry metaclass.
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"""
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if not isinstance(registry_store, dict):
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raise TypeError("'registry_store' argument must be a dict")
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class Registry(type):
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"""
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A metaclass that stores a reference to all registered classes.
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"""
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def __new__(meta, class_name, base_classes, class_dict):
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"""
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Creates and returns a new instance of Registry, which is a
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class named 'class_name' derived from 'base_classes' that
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defines 'class_dict' as additional attributes.
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The returned class is added to 'registry_store' using
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class_dict["REGISTERED_NAME"] as the name, or 'class_name'
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if the "REGISTERED_NAME" attribute isn't defined. If the
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sentinel value 'LEAVE_UNREGISTERED' is specified as the
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name, then the returned class isn't added to
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'registry_store'.
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The returned class will have the "REGISTERED_NAME" attribute
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defined either as its associated key in 'registry_store' or
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the 'LEAVE_UNREGISTERED' sentinel value.
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"""
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registered_name = class_dict.setdefault("REGISTERED_NAME", class_name)
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cls = type.__new__(meta, class_name, base_classes, class_dict)
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if registered_name is not LEAVE_UNREGISTERED:
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if registered_name in registry_store:
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raise ValueError(
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"The name %s is already registered; a different value for the"
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" 'REGISTERED_NAME' attribute must be chosen" % (registered_name))
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registry_store[registered_name] = cls
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return cls
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return Registry
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