Update _can_assign_attr to return False for builtins.object (#946)

* Add tests of expected behaviour for delayed attr assign

Ref #945

* Update _can_assign_attr to return False for builtins.object

Ref #945, pylint#4232, pylint#3970, pylint#3595. Various interactions
had been previously noticed with the typing/collections modules and
methods named prev/next on objects. This was due to inference setting
these values as instance attributes on the builtin object class due to
a sentinel object and an incorrectly inferred return value in the
OrderedDict definition. This change updates _can_assign_attr (and the
resulting delayed_assattr behaviour) to ignore attempts to assign to
object() since these would fail.

* Fix test definition for MRO inference

* Update changelog

ChangeLog

@@ -6,6 +6,15 @@ What's New in astroid 2.6.0?
 ============================
 Release Date: TBA
 
+* Do not set instance attributes on builtin object()
+
+ Closes #945
+ Closes PyCQA/pylint#4232
+ Closes PyCQA/pylint#4221
+ Closes PyCQA/pylint#3970
+ Closes PyCQA/pylint#3595
+
+
 What's New in astroid 2.5.6?
 ============================
 Release Date: 2021-04-25

astroid/builder.py

@@ -67,7 +67,7 @@ def _can_assign_attr(node, attrname):
     else:
         if slots and attrname not in {slot.value for slot in slots}:
             return False
-    return True
+    return node.qname() != "builtins.object"
 
 
 class AstroidBuilder(raw_building.InspectBuilder):

tests/unittest_builder.py

@@ -28,7 +28,7 @@
 
 import pytest
 
-from astroid import builder, exceptions, manager, nodes, test_utils, util
+from astroid import Instance, builder, exceptions, manager, nodes, test_utils, util
 
 from . import resources
 
@@ -476,6 +476,53 @@ def A_assign_type(self):
         self.assertIn("assign_type", lclass.locals)
         self.assertIn("type", lclass.locals)
 
+    def test_infer_can_assign_regular_object(self):
+        mod = builder.parse(
+            """
+            class A:
+                pass
+            a = A()
+            a.value = "is set"
+            a.other = "is set"
+        """
+        )
+        obj = list(mod.igetattr("a"))
+        self.assertEqual(len(obj), 1)
+        obj = obj[0]
+        self.assertIsInstance(obj, Instance)
+        self.assertIn("value", obj.instance_attrs)
+        self.assertIn("other", obj.instance_attrs)
+
+    def test_infer_can_assign_has_slots(self):
+        mod = builder.parse(
+            """
+            class A:
+                __slots__ = ('value',)
+            a = A()
+            a.value = "is set"
+            a.other = "not set"
+        """
+        )
+        obj = list(mod.igetattr("a"))
+        self.assertEqual(len(obj), 1)
+        obj = obj[0]
+        self.assertIsInstance(obj, Instance)
+        self.assertIn("value", obj.instance_attrs)
+        self.assertNotIn("other", obj.instance_attrs)
+
+    def test_infer_can_assign_no_classdict(self):
+        mod = builder.parse(
+            """
+            a = object()
+            a.value = "not set"
+        """
+        )
+        obj = list(mod.igetattr("a"))
+        self.assertEqual(len(obj), 1)
+        obj = obj[0]
+        self.assertIsInstance(obj, Instance)
+        self.assertNotIn("value", obj.instance_attrs)
+
     def test_augassign_attr(self):
         builder.parse(
             """

tests/unittest_scoped_nodes.py

@@ -1430,7 +1430,9 @@ class A:
             pass
         class B:
             pass
-        scope = object()
+        class Scope:
+            pass
+        scope = Scope()
         scope.A = A
         scope.B = B
         class C(scope.A, scope.B):