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54 lines
1.8 KiB
Python
54 lines
1.8 KiB
Python
import weakref
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from abc import ABCMeta, abstractmethod
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from typing import Any, ClassVar, Generic, MutableMapping, TypeVar
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from typing_extensions import override
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_KeyT = TypeVar("_KeyT")
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_DataT = TypeVar("_DataT")
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class BaseNotes(Generic[_KeyT, _DataT], metaclass=ABCMeta):
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"""This class defines a generic type-safe mechanism for associating
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additional data with an object (without modifying the original
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object via subclassing or monkey-patching).
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It was originally designed to avoid monkey-patching the Django
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HttpRequest object, to which we want to associate computed state
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(e.g. parsed state computed from the User-Agent) so that it's
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available in code paths that receive the HttpRequest object.
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The implementation uses a WeakKeyDictionary, so that the notes
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object will be garbage-collected when the original object no
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longer has other references (avoiding memory leaks).
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We still need to be careful to avoid any of the attributes of
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_DataT having points to the original object, as that can create a
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cyclic reference cycle that the Python garbage collect may not
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handle correctly.
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"""
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__notes_map: ClassVar[MutableMapping[Any, Any]]
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@override
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def __init_subclass__(cls, **kwargs: object) -> None:
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super().__init_subclass__(**kwargs)
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if not hasattr(cls, "__notes_map"):
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cls.__notes_map = weakref.WeakKeyDictionary()
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@classmethod
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def get_notes(cls, key: _KeyT) -> _DataT:
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try:
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return cls.__notes_map[key]
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except KeyError:
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cls.__notes_map[key] = cls.init_notes()
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return cls.__notes_map[key]
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@classmethod
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def set_notes(cls, key: _KeyT, notes: _DataT) -> None:
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cls.__notes_map[key] = notes
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@classmethod
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@abstractmethod
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def init_notes(cls) -> _DataT: ...
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