https://github.com/JetBrains/intellij-community/pull/149

python/src/com/jetbrains/python/psi/PyUtil.java

@@ -51,7 +51,6 @@ import com.intellij.util.IncorrectOperationException;
 import com.intellij.util.PlatformIcons;
 import com.intellij.util.SmartList;
 import com.intellij.util.containers.ContainerUtil;
-import com.intellij.util.containers.HashSet;
 import com.jetbrains.python.PyBundle;
 import com.jetbrains.python.PyNames;
 import com.jetbrains.python.PyTokenTypes;
@@ -964,6 +963,17 @@ public class PyUtil {
     while (value instanceof PyParenthesizedExpression) {
       value = ((PyParenthesizedExpression)value).getContainedExpression();
     }
+    if (value instanceof PyReferenceExpression) {
+      PyReferenceExpression refExpr = (PyReferenceExpression)value;
+      PsiElement deref = refExpr.getReference().resolve();
+      if (deref instanceof PyTargetExpression) {
+        PyTargetExpression te = (PyTargetExpression)deref;
+        PyExpression assignedValue = te.findAssignedValue();
+        if (assignedValue instanceof PySequenceExpression) {
+          value = assignedValue;
+        }
+      }
+    }
     if (value instanceof PySequenceExpression) {
       final PyExpression[] elements = ((PySequenceExpression)value).getElements();
       List<String> result = new ArrayList<String>(elements.length);

python/testData/inspections/PyUnresolvedReferencesInspection/slotsAsExternal.py

@@ -0,0 +1,25 @@
+class A(object):
+    __slots__ = ['a', 'b']
+
+    def __init__(self):
+        self.a = None  # <- all ok here
+        self.b = None  # <- all ok here
+
+
+class B(A):
+    __slots__ = A.__slots__
+
+    def __init__(self):
+        super(B, self).__init__()
+        self.<warning descr="'B' object has no attribute 'c'">c</warning> = 'bug'
+
+
+EXTERNAL_SLOTS = ['a', 'b']
+
+
+class C(A):
+    __slots__ = EXTERNAL_SLOTS
+
+    def __init__(self):
+        super(C, self).__init__()
+        self.<warning descr="'C' object has no attribute 'c'">c</warning> = 'bug'

python/testSrc/com/jetbrains/python/inspections/PyUnresolvedReferencesInspectionTest.java

@@ -52,6 +52,10 @@ public class PyUnresolvedReferencesInspectionTest extends PyInspectionTestCase {
     doTest();
   }
 
+  public void testSlotsAsExternal() {
+    doTest();
+  }
+
   public void testImportExceptImportError() {
     doTest();
   }