Extract with assignment for index assignment

Very similar to the case resolved in #391 by @climbus, but for index
assignment instead of attribute assignment.

rope/refactor/extract.py

@@ -312,7 +312,7 @@ def _find_matches(self, collector):
 
     @staticmethod
     def _is_assignment(region_match):
-        return isinstance(region_match.ast, ast.Attribute) and isinstance(region_match.ast.ctx, ast.Store)
+        return isinstance(region_match.ast, (ast.Attribute, ast.Subscript)) and isinstance(region_match.ast.ctx, ast.Store)
 
     def _where_to_search(self):
         if self.info.similar:

ropetest/refactor/extracttest.py

@@ -1811,7 +1811,7 @@ def second_method(someargs):
 
         self.assertEqual(expected, refactored)
 
-    def test_extract_function_expression_with_assignment(self):
+    def test_extract_function_expression_with_assignment_to_attribute(self):
         code = dedent('''\
             class A(object):
                 def func(self):
@@ -1833,6 +1833,28 @@ def new_func(self):
 
         self.assertEqual(expected, refactored)
 
+    def test_extract_function_expression_with_assignment_index(self):
+        code = dedent('''\
+            class A(object):
+                def func(self, val):
+                    self[val] = 1
+                    var_bb = self[val]
+        ''')
+        extract_target = '= self[val]'
+        start, end = code.index(extract_target)+2, code.index(extract_target)+2 + len(extract_target) - 2
+        refactored = self.do_extract_method(code, start, end, 'new_func', similar=True)
+        expected = dedent('''\
+            class A(object):
+                def func(self, val):
+                    self[val] = 1
+                    var_bb = self.new_func(val)
+
+                def new_func(self, val):
+                    return self[val]
+        ''')
+
+        self.assertEqual(expected, refactored)
+
     def test_extraction_method_with_global_variable(self):
         code = dedent('''\
             g = None