searxng/searx/compat.py

# SPDX-License-Identifier: AGPL-3.0-or-later
# lint: pylint
# pyright: basic
"""Module for backward compatibility.

"""
# pylint: disable=C,R


__all__ = ('cached_property',)


try:
    from functools import cached_property  # type: ignore

except ImportError:

    # cache_property has been added in py3.8 [1]
    #
    # To support cache_property in py3.7 the implementation from 3.8 has been
    # copied here.  This code can be cleanup with EOL of py3.7.
    #
    # [1] https://docs.python.org/3/library/functools.html#functools.cached_property

    from threading import RLock

    _NOT_FOUND = object()

    class cached_property:
        def __init__(self, func):
            self.func = func
            self.attrname = None
            self.__doc__ = func.__doc__
            self.lock = RLock()

        def __set_name__(self, owner, name):
            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):
            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