Preserve AttributeError in slotted classes with cached_property (#1253)

* Preserve AttributeError in slotted classes with cached_property

In slotted classes' generated __getattr__(), we try __getattribute__()
before __getattr__(), if available, and eventually let AttributeError
propagate. This matches better with the behaviour described in Python's
documentation "Customizing attribute access":

  https://docs.python.org/3/reference/datamodel.html#customizing-attribute-access

Fix https://github.com/python-attrs/attrs/issues/1230

* Update changelog.d/1253.change.md

---------

Co-authored-by: Hynek Schlawack <hs@ox.cx>

changelog.d/1253.change.md

@@ -0,0 +1 @@
+Preserve `AttributeError` raised by properties of slotted classes with `functools.cached_properties`.

src/attr/_make.py

@@ -619,7 +619,11 @@ def _make_cached_property_getattr(cached_properties, original_getattr, cls):
     else:
         lines.extend(
             [
-                "         if hasattr(super(), '__getattr__'):",
+                "         try:",
+                "             return super().__getattribute__(item)",
+                "         except AttributeError:",
+                "             if not hasattr(super(), '__getattr__'):",
+                "                 raise",
                 "             return super().__getattr__(item)",
                 "         original_error = f\"'{self.__class__.__name__}' object has no attribute '{item}'\"",
                 "         raise AttributeError(original_error)",

tests/test_slots.py

@@ -806,6 +806,45 @@ def test_slots_cached_property_with_empty_getattr_raises_attribute_error_of_requ
         a.z
 
 
+@pytest.mark.skipif(not PY_3_8_PLUS, reason="cached_property is 3.8+")
+def test_slots_cached_property_raising_attributeerror():
+    """
+    Ensures AttributeError raised by a property is preserved by __getattr__()
+    implementation.
+
+    Regression test for issue https://github.com/python-attrs/attrs/issues/1230
+    """
+
+    @attr.s(slots=True)
+    class A:
+        x = attr.ib()
+
+        @functools.cached_property
+        def f(self):
+            return self.p
+
+        @property
+        def p(self):
+            raise AttributeError("I am a property")
+
+        @functools.cached_property
+        def g(self):
+            return self.q
+
+        @property
+        def q(self):
+            return 2
+
+    a = A(1)
+    with pytest.raises(AttributeError, match=r"^I am a property$"):
+        a.p
+    with pytest.raises(AttributeError, match=r"^I am a property$"):
+        a.f
+
+    assert a.g == 2
+    assert a.q == 2
+
+
 @pytest.mark.skipif(not PY_3_8_PLUS, reason="cached_property is 3.8+")
 def test_slots_cached_property_with_getattr_calls_getattr_for_missing_attributes():
     """