PY-995 fix introduce* incorrectly working with assignment left part

python/src/com/jetbrains/python/refactoring/introduce/IntroduceHandler.java

@@ -175,7 +175,10 @@ abstract public class IntroduceHandler implements RefactoringActionHandler {
       return;
     }
 
-    final PyExpression expression = (PyExpression)element;
+    final PsiElement parent = element.getParent();
+    final PyExpression expression = parent instanceof PyAssignmentStatement ?
+                                    ((PyAssignmentStatement)parent).getAssignedValue() :
+                                    (PyExpression)element;
 
     final List<PsiElement> occurrences;
     if (expression.getUserData(PyPsiUtils.SELECTION_BREAKS_AST_NODE) == null && !(expression instanceof PyCallExpression)) {
@@ -185,6 +188,7 @@ abstract public class IntroduceHandler implements RefactoringActionHandler {
       occurrences = Collections.emptyList();
     }
     String[] possibleNames = getSuggestedNames(expression);
+    replaceAll &= occurrences.size() > 0;
     boolean initInConstructor = false;
     if (name == null) {
       PyIntroduceDialog dialog = new PyIntroduceDialog(project, expression, myDialogTitle, myValidator, occurrences.size(), possibleNames, getHelpId(), hasConstructor);

python/testData/refactoring/introduceVariable/py995.after.py

@@ -0,0 +1,4 @@
+class ConferenceTest(TestCase):
+    def testSimple(self):
+        a = Conference()
+        c = a

python/testData/refactoring/introduceVariable/py995.py

@@ -0,0 +1,3 @@
+class ConferenceTest(TestCase):
+    def testSimple(self):
+        <selection>c</selection> = Conference()

python/testSrc/com/jetbrains/python/refactoring/PyIntroduceVariableTest.java

@@ -13,9 +13,17 @@ public class PyIntroduceVariableTest extends PyLightFixtureTestCase {
   }
 
   public void testSimple() throws Exception {
-    myFixture.configureByFile("simple.py");
+    doTest();
+  }
+
+  public void testPy995() throws Exception {
+    doTest();
+  }
+
+  private void doTest() throws Exception {
+    myFixture.configureByFile(getTestName(true) + ".py");
     VariableIntroduceHandler handler = new VariableIntroduceHandler();
     handler.performAction(myFixture.getProject(),  myFixture.getEditor(), myFixture.getFile(), "a", true, false);
-    myFixture.checkResultByFile("simple.after.py");
+    myFixture.checkResultByFile(getTestName(true) + ".after.py");
   }
 }