BUG: .unique() on MultiIndex: preserve names

closes pandas-dev#20568
closes pandas-dev#20570

Author: toobaz

doc/source/whatsnew/v0.23.0.txt

@@ -1069,6 +1069,8 @@ MultiIndex
 - Bug in :func:`MultiIndex.__contains__` where non-tuple keys would return ``True`` even if they had been dropped (:issue:`19027`)
 - Bug in :func:`MultiIndex.set_labels` which would cause casting (and potentially clipping) of the new labels if the ``level`` argument is not 0 or a list like [0, 1, ... ]  (:issue:`19057`)
 - Bug in :func:`MultiIndex.get_level_values` which would return an invalid index on level of ints with missing values (:issue:`17924`)
+- Bug in :func:`MultiIndex.unique` when called on empty :class:`MultiIndex` (:issue:`20568`)
+- Bug in :func:`MultiIndex.unique` which would not preserve level names (:issue:`20570`)
 - Bug in :func:`MultiIndex.remove_unused_levels` which would fill nan values (:issue:`18417`)
 - Bug in :func:`MultiIndex.from_tuples` which would fail to take zipped tuples in python3 (:issue:`18434`)
 - Bug in :func:`MultiIndex.get_loc` which would fail to automatically cast values between float and int (:issue:`18818`, :issue:`15994`)

pandas/core/indexes/multi.py

@@ -553,11 +553,10 @@ def __contains__(self, key):
     @Appender(_index_shared_docs['_shallow_copy'])
     def _shallow_copy(self, values=None, **kwargs):
         if values is not None:
-            if 'name' in kwargs:
-                kwargs['names'] = kwargs.pop('name', None)
+            names = kwargs.pop('names', kwargs.pop('name', self.names))
             # discards freq
             kwargs.pop('freq', None)
-            return MultiIndex.from_tuples(values, **kwargs)
+            return MultiIndex.from_tuples(values, names=names, **kwargs)
         return self.view()
 
     @cache_readonly

pandas/tests/indexes/test_multi.py

@@ -2453,22 +2453,31 @@ def test_get_unique_index(self):
             tm.assert_index_equal(result, expected)
 
     def test_unique(self):
-        mi = pd.MultiIndex.from_arrays([[1, 2, 1, 2], [1, 1, 1, 2]])
+        mi = pd.MultiIndex.from_arrays([[1, 2, 1, 2], [1, 1, 1, 2]],
+                                       names=['first', 'second'])
 
         res = mi.unique()
-        exp = pd.MultiIndex.from_arrays([[1, 2, 2], [1, 1, 2]])
+        exp = pd.MultiIndex.from_arrays([[1, 2, 2], [1, 1, 2]], names=mi.names)
         tm.assert_index_equal(res, exp)
 
-        mi = pd.MultiIndex.from_arrays([list('aaaa'), list('abab')])
+        mi = pd.MultiIndex.from_arrays([list('aaaa'), list('abab')],
+                                       names=['first', 'second'])
         res = mi.unique()
-        exp = pd.MultiIndex.from_arrays([list('aa'), list('ab')])
+        exp = pd.MultiIndex.from_arrays([list('aa'), list('ab')],
+                                        names=mi.names)
         tm.assert_index_equal(res, exp)
 
-        mi = pd.MultiIndex.from_arrays([list('aaaa'), list('aaaa')])
+        mi = pd.MultiIndex.from_arrays([list('aaaa'), list('aaaa')],
+                                       names=['first', 'second'])
         res = mi.unique()
-        exp = pd.MultiIndex.from_arrays([['a'], ['a']])
+        exp = pd.MultiIndex.from_arrays([['a'], ['a']], names=mi.names)
         tm.assert_index_equal(res, exp)
 
+        # GH #20568 - empty MI
+        mi = pd.MultiIndex.from_arrays([[], []], names=['first', 'second'])
+        res = mi.unique()
+        tm.assert_index_equal(mi, res)
+
     @pytest.mark.parametrize('level', [0, 'first', 1, 'second'])
     def test_unique_level(self, level):
         # GH #17896 - with level= argument
@@ -2483,6 +2492,11 @@ def test_unique_level(self, level):
         expected = mi.get_level_values(level)
         tm.assert_index_equal(result, expected)
 
+        # With empty MI
+        mi = pd.MultiIndex.from_arrays([[], []], names=['first', 'second'])
+        result = mi.unique(level=level)
+        expected = mi.get_level_values(level)
+
     def test_unique_datetimelike(self):
         idx1 = pd.DatetimeIndex(['2015-01-01', '2015-01-01', '2015-01-01',
                                  '2015-01-01', 'NaT', 'NaT'])