Source code for backports.cached_property

"""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
try:
    # Local Implementation
    from ._version import version as __version__
except ImportError:
    pass

if version_info >= (3, 8):
    # Standard Library
    from functools import cached_property  # pylint: disable=no-name-in-module
else:
    # Standard Library
    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
    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.
        """

        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: Optional[_S], owner: Optional[Type[Any]] = 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