Deprecated Index.get_duplicates() (#20544)

doc/source/whatsnew/v0.23.0.txt

@@ -886,6 +886,7 @@ Deprecations
 - :func:`Series.rolling().apply() <pandas.core.window.Rolling.apply>`, :func:`DataFrame.rolling().apply() <pandas.core.window.Rolling.apply>`,
   :func:`Series.expanding().apply() <pandas.core.window.Expanding.apply>`, and :func:`DataFrame.expanding().apply() <pandas.core.window.Expanding.apply>` have deprecated passing an ``np.array`` by default. One will need to pass the new ``raw`` parameter to be explicit about what is passed (:issue:`20584`)
 - ``DatetimeIndex.offset`` is deprecated. Use ``DatetimeIndex.freq`` instead (:issue:`20716`)
+- ``Index.get_duplicates()`` is deprecated and will be removed in a future version (:issue:`20239`)
 
 .. _whatsnew_0230.prior_deprecations:
 

pandas/core/frame.py

@@ -3879,7 +3879,7 @@ def set_index(self, keys, drop=True, append=False, inplace=False,
         index = _ensure_index_from_sequences(arrays, names)
 
         if verify_integrity and not index.is_unique:
-            duplicates = index.get_duplicates()
+            duplicates = index[index.duplicated()].unique()
             raise ValueError('Index has duplicate keys: {dup}'.format(
                 dup=duplicates))
 

pandas/core/indexes/base.py

@@ -1851,6 +1851,9 @@ def get_duplicates(self):
         Returns a sorted list of index elements which appear more than once in
         the index.
 
+        .. deprecated:: 0.23.0
+            Use idx[idx.duplicated()].unique() instead
+
         Returns
         -------
         array-like
@@ -1897,13 +1900,12 @@ def get_duplicates(self):
         >>> pd.Index(dates).get_duplicates()
         DatetimeIndex([], dtype='datetime64[ns]', freq=None)
         """
-        from collections import defaultdict
-        counter = defaultdict(lambda: 0)
-        for k in self.values:
-            counter[k] += 1
-        return sorted(k for k, v in compat.iteritems(counter) if v > 1)
+        warnings.warn("'get_duplicates' is deprecated and will be removed in "
+                      "a future release. You can use "
+                      "idx[idx.duplicated()].unique() instead",
+                      FutureWarning, stacklevel=2)
 
-    _get_duplicates = get_duplicates
+        return self[self.duplicated()].unique()
 
     def _cleanup(self):
         self._engine.clear_mapping()

pandas/core/indexes/datetimelike.py

@@ -502,10 +502,6 @@ def take(self, indices, axis=0, allow_fill=True,
         freq = self.freq if isinstance(self, ABCPeriodIndex) else None
         return self._shallow_copy(taken, freq=freq)
 
-    def get_duplicates(self):
-        values = Index.get_duplicates(self)
-        return self._simple_new(values)
-
     _can_hold_na = True
 
     _na_value = NaT

pandas/core/reshape/concat.py

@@ -504,7 +504,7 @@ def _get_concat_axis(self):
     def _maybe_check_integrity(self, concat_index):
         if self.verify_integrity:
             if not concat_index.is_unique:
-                overlap = concat_index.get_duplicates()
+                overlap = concat_index[concat_index.duplicated()].unique()
                 raise ValueError('Indexes have overlapping values: '
                                  '{overlap!s}'.format(overlap=overlap))
 

pandas/tests/indexes/datetimes/test_datetime.py

@@ -1,3 +1,4 @@
+import warnings
 
 import pytest
 
@@ -178,7 +179,10 @@ def test_get_duplicates(self):
         idx = DatetimeIndex(['2000-01-01', '2000-01-02', '2000-01-02',
                              '2000-01-03', '2000-01-03', '2000-01-04'])
 
-        result = idx.get_duplicates()
+        with warnings.catch_warnings(record=True):
+            # Deprecated - see GH20239
+            result = idx.get_duplicates()
+
         ex = DatetimeIndex(['2000-01-02', '2000-01-03'])
         tm.assert_index_equal(result, ex)
 

pandas/tests/indexes/test_base.py

@@ -2068,6 +2068,11 @@ def test_cached_properties_not_settable(self):
         with tm.assert_raises_regex(AttributeError, "Can't set attribute"):
             idx.is_unique = False
 
+    def test_get_duplicates_deprecated(self):
+        idx = pd.Index([1, 2, 3])
+        with tm.assert_produces_warning(FutureWarning):
+            idx.get_duplicates()
+
 
 class TestMixedIntIndex(Base):
     # Mostly the tests from common.py for which the results differ

pandas/tests/indexes/test_multi.py

@@ -2432,7 +2432,12 @@ def check(nlevels, with_nulls):
         for a in [101, 102]:
             mi = MultiIndex.from_arrays([[101, a], [3.5, np.nan]])
             assert not mi.has_duplicates
-            assert mi.get_duplicates() == []
+
+            with warnings.catch_warnings(record=True):
+                # Deprecated - see GH20239
+                assert mi.get_duplicates().equals(MultiIndex.from_arrays(
+                    [[], []]))
+
             tm.assert_numpy_array_equal(mi.duplicated(), np.zeros(
                 2, dtype='bool'))
 
@@ -2444,7 +2449,12 @@ def check(nlevels, with_nulls):
                                 labels=np.random.permutation(list(lab)).T)
                 assert len(mi) == (n + 1) * (m + 1)
                 assert not mi.has_duplicates
-                assert mi.get_duplicates() == []
+
+                with warnings.catch_warnings(record=True):
+                    # Deprecated - see GH20239
+                    assert mi.get_duplicates().equals(MultiIndex.from_arrays(
+                        [[], []]))
+
                 tm.assert_numpy_array_equal(mi.duplicated(), np.zeros(
                     len(mi), dtype='bool'))
 

pandas/tests/indexes/timedeltas/test_timedelta.py

@@ -1,3 +1,5 @@
+import warnings
+
 import pytest
 
 import numpy as np
@@ -145,7 +147,10 @@ def test_get_duplicates(self):
         idx = TimedeltaIndex(['1 day', '2 day', '2 day', '3 day', '3day',
                               '4day'])
 
-        result = idx.get_duplicates()
+        with warnings.catch_warnings(record=True):
+            # Deprecated - see GH20239
+            result = idx.get_duplicates()
+
         ex = TimedeltaIndex(['2 day', '3day'])
         tm.assert_index_equal(result, ex)