Don't calculate slots when MRO parsing fails. (#1089)

* Don't calculate slots when MRO parsing fails.

* Added changelog entry.

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

ChangeLog

@@ -44,6 +44,10 @@ Release date: 2021-07-19
 
 * Added ``If.is_sys_guard`` and ``If.is_typing_guard`` helper methods
 
+* Fix handling of classes with duplicated bases with the same name
+
+  Closes PyCQA/astroid#1088
+
 * Fix a bad inferenece type for yield values inside of a derived class.
 
   Closes PyCQA/astroid#1090

astroid/scoped_nodes.py

@@ -42,6 +42,7 @@
 import builtins
 import io
 import itertools
+import typing
 from typing import List, Optional
 
 from astroid import bases
@@ -2906,9 +2907,11 @@ def slots(self):
         :rtype: list(str) or None
         """
 
-        def grouped_slots():
+        def grouped_slots(
+            mro: List["ClassDef"],
+        ) -> typing.Iterator[Optional[node_classes.NodeNG]]:
             # Not interested in object, since it can't have slots.
-            for cls in self.mro()[:-1]:
+            for cls in mro[:-1]:
                 try:
                     cls_slots = cls._slots()
                 except NotImplementedError:
@@ -2923,7 +2926,14 @@ def grouped_slots():
                 "The concept of slots is undefined for old-style classes."
             )
 
-        slots = list(grouped_slots())
+        try:
+            mro = self.mro()
+        except MroError as e:
+            raise NotImplementedError(
+                "Cannot get slots while parsing mro fails."
+            ) from e
+
+        slots = list(grouped_slots(mro))
         if not all(slot is not None for slot in slots):
             return None
 

tests/unittest_scoped_nodes.py

@@ -2230,5 +2230,16 @@ class C(B[str]): pass
     ]
 
 
+def test_slots_duplicate_bases_issue_1089():
+    astroid = builder.parse(
+        """
+            class First(object, object): #@
+                pass
+        """
+    )
+    with pytest.raises(NotImplementedError):
+        astroid["First"].slots()
+
+
 if __name__ == "__main__":
     unittest.main()