Fixed #28269 -- Fixed Model.__init__() crash on models with a field that has an instance only descriptor.

Regression in d2a26c1a90.

django/db/models/options.py

@@ -1,4 +1,5 @@
 import copy
+import inspect
 import warnings
 from bisect import bisect
 from collections import OrderedDict, defaultdict
@@ -829,7 +830,9 @@ class Options:
     @cached_property
     def _property_names(self):
         """Return a set of the names of the properties defined on the model."""
-        return frozenset({
-            attr for attr in
-            dir(self.model) if isinstance(getattr(self.model, attr), property)
-        })
+        names = []
+        for name in dir(self.model):
+            attr = inspect.getattr_static(self.model, name)
+            if isinstance(attr, property):
+                names.append(name)
+        return frozenset(names)

docs/releases/1.11.3.txt

@@ -12,3 +12,6 @@ Bugfixes
 * Removed an incorrect deprecation warning about a missing ``renderer``
   argument if a ``Widget.render()`` method accepts ``**kwargs``
   (:ticket:`28265`).
+
+* Fixed a regression causing ``Model.__init__()`` to crash if a field has an
+  instance only descriptor (:ticket:`28269`).

tests/model_meta/models.py

@@ -9,6 +9,13 @@ class Relation(models.Model):
     pass
 
 
+class InstanceOnlyDescriptor(object):
+    def __get__(self, instance, cls=None):
+        if instance is None:
+            raise AttributeError('Instance only')
+        return 1
+
+
 class AbstractPerson(models.Model):
     # DATA fields
     data_abstract = models.CharField(max_length=10)
@@ -43,6 +50,8 @@ class AbstractPerson(models.Model):
     def test_property(self):
         return 1
 
+    test_instance_only_descriptor = InstanceOnlyDescriptor()
+
 
 class BasePerson(AbstractPerson):
     # DATA fields

tests/model_meta/tests.py

@@ -276,4 +276,6 @@ class ParentListTests(SimpleTestCase):
 
 class PropertyNamesTests(SimpleTestCase):
     def test_person(self):
+        # Instance only descriptors don't appear in _property_names.
+        self.assertEqual(AbstractPerson().test_instance_only_descriptor, 1)
         self.assertEqual(AbstractPerson._meta._property_names, frozenset(['pk', 'test_property']))