paquo.utils: fix ro cached_property implementation for py36/py37

paquo/_utils.py

@@ -13,18 +13,27 @@ def __set__(self, obj, value):
 except ImportError:
     # noinspection PyPep8Naming
     class cached_property:  # type: ignore  # https://github.com/python/mypy/issues/1153
-        def __init__(self, getter):
-            self.getter = getter
-            self.name = getter.__name__
+        _NOCACHE = object()
+
+        def __init__(self, fget):
+            self.fget = fget
+            self.attrname = None
+            self.__doc__ = fget.__doc__
+
+        def __set_name__(self, owner, name):
+            self.attrname = name
 
         def __get__(self, obj, objtype=None):
             if obj is None:
                 return self  # pragma: no cover
-            value = obj.__dict__[self.name] = self.getter(obj)
-            return value
+            cache = obj.__dict__
+            val = cache.get(self.attrname, self._NOCACHE)
+            if val is self._NOCACHE:
+                val = cache[self.attrname] = self.fget(obj)
+            return val
 
-        # def __set__(self, obj, value):
-        #     raise AttributeError(f"readonly attribute {self.name}")
+        def __set__(self, obj, value):
+            raise AttributeError(f"readonly attribute {self.fget.__name__}")
 
 try:
     from contextlib import nullcontext  # type: ignore