move block to _shallow_copy_with_infer

doc/source/whatsnew/v0.20.2.txt

@@ -40,7 +40,7 @@ Bug Fixes
 - Silenced a warning on some Windows environments about "tput: terminal attributes: No such device or address" when
   detecting the terminal size. This fix only applies to python 3 (:issue:`16496`)
 - Bug in using ``pathlib.Path`` or ``py.path.local`` objects with io functions (:issue:`16291`)
-- Bug in ``Index`` calling symmetric_difference() on two equal multiindices results in a TypeError (:issue `13490`)
+- Bug in ``Index.symmetric_difference()`` on two equal MultiIndex's, results in a TypeError (:issue `13490`)
 - Bug in ``DataFrame.update()`` with ``overwrite=False`` and ``NaN values`` (:issue:`15593`)
 
 

pandas/core/indexes/multi.py

@@ -414,16 +414,16 @@ def view(self, cls=None):
         return result
 
     def _shallow_copy_with_infer(self, values=None, **kwargs):
+        # On equal MultiIndexes the difference is empty.
+        # Therefore, an empty MultiIndex is returned GH13490
+        if len(values) == 0:
+            return MultiIndex(levels=[[] for _ in range(self.nlevels)],
+                              labels=[[] for _ in range(self.nlevels)], **kwargs)
         return self._shallow_copy(values, **kwargs)
 
     @Appender(_index_shared_docs['_shallow_copy'])
     def _shallow_copy(self, values=None, **kwargs):
         if values is not None:
-            # On equal MultiIndexes the difference is empty.
-            # Therefore, an empty MultiIndex is returned GH13490
-            if len(values) == 0:
-                return MultiIndex(levels=[[] for _ in range(self.nlevels)],
-                                  labels=[[] for _ in range(self.nlevels)], **kwargs)
             if 'name' in kwargs:
                 kwargs['names'] = kwargs.pop('name', None)
             # discards freq