"""
MIT License
Copyright (c) 2020 Alexey Stepanov
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
Backport of python 3.8 functools.cached_property.
cached_property() - computed once per instance, cached as attribute
"""
__all__ = ("cached_property",)
# Standard Library
from sys import version_info
if version_info >= (3, 8):
from functools import cached_property # pylint: disable=no-name-in-module
else:
from threading import RLock
from typing import Any
from typing import Callable
from typing import Optional
from typing import Type
from typing import TypeVar
_NOT_FOUND = object()
_T = TypeVar("_T")
_S = TypeVar("_S")
# noinspection PyPep8Naming
[docs] class cached_property: # NOSONAR # pylint: disable=invalid-name # noqa: N801
"""Cached property implementation.
Transform a method of a class into a property whose value is computed once
and then cached as a normal attribute for the life of the instance.
Similar to property(), with the addition of caching.
Useful for expensive computed properties of instances
that are otherwise effectively immutable.
"""
[docs] def __init__(self, func: Callable[[Any], _T]) -> None:
"""Cached property implementation."""
self.func = func
self.attrname: Optional[str] = None
self.__doc__ = func.__doc__
self.lock = RLock()
def __set_name__(self, owner: Type[Any], name: str) -> None:
"""Assign attribute name and owner."""
if self.attrname is None:
self.attrname = name
elif name != self.attrname:
raise TypeError(
"Cannot assign the same cached_property to two different names "
f"({self.attrname!r} and {name!r})."
)
def __get__(self, instance, owner=None) -> Any:
"""Property-like getter implementation.
:return: property instance if requested on class or value/cached value if requested on instance.
:rtype: Union[cached_property[_T], _T]
:raises TypeError: call without calling __set_name__ or no '__dict__' attribute
"""
if instance is None:
return self
if self.attrname is None:
raise TypeError("Cannot use cached_property instance without calling __set_name__ on it.")
try:
cache = instance.__dict__
except AttributeError: # not all objects have __dict__ (e.g. class defines slots)
msg = (
f"No '__dict__' attribute on {type(instance).__name__!r} "
f"instance to cache {self.attrname!r} property."
)
raise TypeError(msg) from None
val = cache.get(self.attrname, _NOT_FOUND)
if val is _NOT_FOUND:
with self.lock:
# check if another thread filled cache while we awaited lock
val = cache.get(self.attrname, _NOT_FOUND)
if val is _NOT_FOUND:
val = self.func(instance)
try:
cache[self.attrname] = val
except TypeError:
msg = (
f"The '__dict__' attribute on {type(instance).__name__!r} instance "
f"does not support item assignment for caching {self.attrname!r} property."
)
raise TypeError(msg) from None
return val