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from typing import Any, Callable, Optional
__all__ = ("lazy_class_attribute",)
class LazyClassAttribute:
"""Descriptor decorator implementing a class-level, read-only
property, which caches its results on the class(es) on which it
operates.
"""
__slots__ = ("func", "name", "called", "forward_value")
def __init__(
self,
func: Callable[..., Any],
name: Optional[str] = None,
forward_value: Any = None,
):
self.func = func
self.name = name
self.called = False
self.forward_value = forward_value
def __get__(self, instance, cls=None):
if not cls:
cls = type(instance)
# avoid recursion
if self.called:
return self.forward_value
self.called = True
setattr(cls, self.name, self.func())
# "getattr" is used to handle bounded methods
return getattr(cls, self.name)
def __set_name__(self, owner, name):
self.name = self.name or name
lazy_class_attribute = LazyClassAttribute
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