Clearer error message when __dict__ is not mutable.

ncoghlan · Aug 28, 2018 · May 19, 2018 · May 20, 2018 · May 20, 2018 · May 22, 2018
commit 5bdeaab770692de4cb58f43746ba09c024897232
diff --git a/Doc/library/functools.rst b/Doc/library/functools.rst
@@ -45,7 +45,12 @@ The :mod:`functools` module defines the following functions:
 
    .. note::
 
-      This decorator does not support classes which define ``__slots__``.
+      This decorator requires that the ``__dict__`` attribute on each instance
+      be a mutable mapping. This means it will not work with some types, such as
+      metaclasses (since the ``__dict__`` attributes on type instances are
+      read-only proxies for the class namespace), and those that specify
+      ``__slots__`` without including ``__dict__`` as one of the defined slots
+      (as such classes don't provide a ``__dict__`` attribute at all).
 
 
 .. function:: cmp_to_key(func)

diff --git a/Lib/functools.py b/Lib/functools.py
@@ -901,7 +901,7 @@ def __get__(self, instance, owner):
                 "Cannot use cached_property instance without calling __set_name__ on it.")
         try:
             cache = instance.__dict__
-        except AttributeError:  # objects with __slots__ have no __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."
@@ -913,5 +913,13 @@ def __get__(self, instance, owner):
                 # check if another thread filled cache while we awaited lock
                 val = cache.get(self.attrname, _NOT_FOUND)
                 if val is _NOT_FOUND:
-                    val = cache[self.attrname] = self.func(instance)
+                    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
diff --git a/Lib/test/test_functools.py b/Lib/test/test_functools.py
@@ -2406,6 +2406,21 @@ def test_object_with_slots(self):
         ):
             item.cost
 
+    def test_immutable_dict(self):
+        class MyMeta(type):
+            @py_functools.cached_property
+            def prop(self):
+                return True
+
+        class MyClass(metaclass=MyMeta):
+            pass
+
+        with self.assertRaisesRegex(
+            TypeError,
+            "The '__dict__' attribute on 'MyMeta' instance does not support item assignment for caching 'prop' property.",
+        ):
+            MyClass.prop
+
     def test_reuse_different_names(self):
         """Disallow this case because decorated function a would not be cached."""
         with self.assertRaises(RuntimeError) as ctx: