67 lines
2.4 KiB
Python
67 lines
2.4 KiB
Python
from __future__ import annotations
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import typing
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from collections.abc import Callable
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from threading import RLock
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from typing import Any, Generic, TypeVar
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__all__ = ["cached_property"]
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_NOT_FOUND = object()
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_T = TypeVar("_T")
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class cached_property(Generic[_T]):
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def __init__(self, func: Callable[[Any], _T]):
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self.func = func
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self.attrname: str | None = None
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self.__doc__ = func.__doc__
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self.lock = RLock()
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def __set_name__(self, owner: type[Any], name: str) -> None:
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if self.attrname is None:
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self.attrname = name
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elif name != self.attrname:
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msg = f"Cannot assign the same cached_property to two different names ({self.attrname!r} and {name!r})."
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raise TypeError(msg)
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@typing.overload
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def __get__(self, instance: None, owner: type[Any] | None = ...) -> cached_property[_T]:
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...
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@typing.overload
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def __get__(self, instance: object, owner: type[Any] | None = ...) -> _T:
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...
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def __get__(self, instance: object | None, owner: type[Any] | None = None) -> Any:
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if instance is None:
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return self
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if self.attrname is None:
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msg = "Cannot use cached_property instance without calling __set_name__ on it."
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raise TypeError(msg)
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try:
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cache = instance.__dict__
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except AttributeError: # not all objects have __dict__ (e.g. class defines slots)
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msg = (
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f"No '__dict__' attribute on {type(instance).__name__!r} "
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f"instance to cache {self.attrname!r} property."
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)
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raise TypeError(msg) from None
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val = cache.get(self.attrname, _NOT_FOUND)
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if val is _NOT_FOUND:
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with self.lock:
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# check if another thread filled cache while we awaited lock
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val = cache.get(self.attrname, _NOT_FOUND)
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if val is _NOT_FOUND:
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val = self.func(instance)
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try:
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cache[self.attrname] = val
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except TypeError:
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msg = (
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f"The '__dict__' attribute on {type(instance).__name__!r} instance "
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f"does not support item assignment for caching {self.attrname!r} property."
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)
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raise TypeError(msg) from None
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return val
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