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"""Exposes the SubclassRegistry that allows to define a single registration point of one or more subclasses of a |
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(common parent) class. |
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""" |
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from typing import Dict, Generic, TypeVar |
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T = TypeVar('T') |
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class SubclassRegistry(type, Generic[T]): |
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"""Subclass Registry |
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A (parent) class using this class as metaclass gains the 'subclasses' class attribute as well as the 'create' and |
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'register_as_subclass' class methods. |
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The 'subclasses' attribute is a python dictionary having string identifiers as keys and subclasses of the (parent) |
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class as values. |
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The 'register_as_subclass' class method can be used as a decorator to indicate that a (child) class should belong in |
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the parent's class registry. An input string argument will be used as the unique key to register the subclass. |
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The 'create' class method can be invoked with a (string) key and suitable constructor arguments to later construct |
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instances of the corresponding child class. |
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Example: |
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>>> from software_patterns import SubclassRegistry |
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>>> class ClassRegistry(metaclass=SubclassRegistry): |
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... pass |
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>>> ClassRegistry.subclasses |
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{} |
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>>> @ClassRegistry.register_as_subclass('child') |
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... class ChildClass: |
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... def __init__(self, child_attribute): |
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... self.attr = child_attribute |
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>>> child_instance = ClassRegistry.create('child', 'attribute-value') |
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>>> child_instance.attr |
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'attribute-value' |
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>>> type(child_instance).__name__ |
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'ChildClass' |
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>>> isinstance(child_instance, ChildClass) |
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True |
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>>> {k: v.__name__ for k, v in ClassRegistry.subclasses.items()} |
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{'child': 'ChildClass'} |
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""" |
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subclasses: Dict[str, type] |
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def __init__(cls, *args): |
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super().__init__(*args) |
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cls.subclasses = {} |
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def create(cls, subclass_identifier, *args, **kwargs) -> T: |
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"""Create an instance of a registered subclass, given its unique identifier and runtime (constructor) arguments. |
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Invokes the identified subclass constructor passing any supplied arguments. The user needs to know the arguments |
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to supply depending on the resulting constructor signature. |
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Args: |
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subclass_identifier (str): the unique identifier under which to look for the corresponding subclass |
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Raises: |
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UnknownClassError: In case the given identifier is unknown to the parent class |
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InstantiationError: In case the runtime args and kwargs do not match the constructor signature |
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Returns: |
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object: the instance of the registered subclass |
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""" |
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if subclass_identifier not in cls.subclasses: |
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raise UnknownClassError( |
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f'Bad "{str(cls.__name__)}" subclass request; requested subclass with identifier ' |
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f'{str(subclass_identifier)}, but known identifiers are ' |
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f'[{", ".join(str(subclass_id) for subclass_id in cls.subclasses.keys())}]' |
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) |
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try: |
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return cls.subclasses[subclass_identifier](*args, **kwargs) |
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except Exception as any_error: |
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raise InstantiationError( |
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'Error during instance object construction.' |
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f' Failed to create instance of class {cls.subclasses[subclass_identifier]}' |
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f' using args [{", ".join((str(_) for _ in args))}]' |
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f' and kwargs [{", ".join(f"{k}={v}" for k, v in kwargs.items())}]' |
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) from any_error |
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def register_as_subclass(cls, subclass_identifier): |
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"""Register a class as subclass of the parent class. |
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Adds the subclass' constructor in the registry (dict) under the given (str) identifier. Overrides the registry |
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in case of "identifier collision". Can be used as a python decorator. |
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Args: |
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subclass_identifier (str): the user-defined identifier, under which to register the subclass |
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""" |
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def wrapper(subclass): |
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"""Add the (sub) class provided to the parent class registry. |
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Args: |
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subclass ([type]): the (sub) class to register |
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Returns: |
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object: the (sub) class |
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""" |
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cls.subclasses[subclass_identifier] = subclass |
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return subclass |
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return wrapper |
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class InstantiationError(Exception): |
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pass |
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class UnknownClassError(Exception): |
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pass |
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boromir674 /
software-patterns
