DEPR: Index.reindex with non-unique Index (#49485)

Author: jbrockmendel

doc/source/whatsnew/v2.0.0.rst

@@ -250,6 +250,7 @@ Removal of prior version deprecations/changes
 - Disallow passing non-keyword arguments to :meth:`DataFrame.where` and :meth:`Series.where` except for ``cond`` and ``other`` (:issue:`41523`)
 - Disallow passing non-keyword arguments to :meth:`Series.set_axis` and :meth:`DataFrame.set_axis` except for ``labels`` (:issue:`41491`)
 - Disallow passing non-keyword arguments to :meth:`Series.rename_axis` and :meth:`DataFrame.rename_axis` except for ``mapper`` (:issue:`47587`)
+- Disallow :meth:`Index.reindex` with non-unique :class:`Index` objects (:issue:`42568`)
 - Disallow passing non-keyword arguments to :meth:`Series.clip` and :meth:`DataFrame.clip` (:issue:`41511`)
 - Disallow passing non-keyword arguments to :meth:`Series.bfill`, :meth:`Series.ffill`, :meth:`DataFrame.bfill` and :meth:`DataFrame.ffill` (:issue:`41508`)
 - Disallow passing non-keyword arguments to :meth:`DataFrame.replace`, :meth:`Series.replace` except for ``to_replace`` and ``value`` (:issue:`47587`)

pandas/core/indexes/base.py

@@ -4339,12 +4339,7 @@ def reindex(
 
                 if not self.is_unique:
                     # GH#42568
-                    warnings.warn(
-                        "reindexing with a non-unique Index is deprecated and "
-                        "will raise in a future version.",
-                        FutureWarning,
-                        stacklevel=find_stack_level(),
-                    )
+                    raise ValueError("cannot reindex on an axis with duplicate labels")
 
         target = self._wrap_reindex_result(target, indexer, preserve_names)
         return target, indexer

pandas/core/indexes/category.py

@@ -414,12 +414,7 @@ def reindex(
             indexer, missing = self.get_indexer_non_unique(target)
             if not self.is_unique:
                 # GH#42568
-                warnings.warn(
-                    "reindexing with a non-unique Index is deprecated and will "
-                    "raise in a future version.",
-                    FutureWarning,
-                    stacklevel=find_stack_level(),
-                )
+                raise ValueError("cannot reindex on an axis with duplicate labels")
 
         new_target: Index
         if len(self) and indexer is not None:

pandas/tests/indexes/categorical/test_reindex.py

@@ -1,4 +1,5 @@
 import numpy as np
+import pytest
 
 from pandas import (
     Categorical,
@@ -12,37 +13,28 @@
 class TestReindex:
     def test_reindex_list_non_unique(self):
         # GH#11586
+        msg = "cannot reindex on an axis with duplicate labels"
         ci = CategoricalIndex(["a", "b", "c", "a"])
-        with tm.assert_produces_warning(FutureWarning, match="non-unique"):
-            res, indexer = ci.reindex(["a", "c"])
-
-        tm.assert_index_equal(res, Index(["a", "a", "c"]), exact=True)
-        tm.assert_numpy_array_equal(indexer, np.array([0, 3, 2], dtype=np.intp))
+        with pytest.raises(ValueError, match=msg):
+            ci.reindex(["a", "c"])
 
     def test_reindex_categorical_non_unique(self):
+        msg = "cannot reindex on an axis with duplicate labels"
         ci = CategoricalIndex(["a", "b", "c", "a"])
-        with tm.assert_produces_warning(FutureWarning, match="non-unique"):
-            res, indexer = ci.reindex(Categorical(["a", "c"]))
-
-        exp = CategoricalIndex(["a", "a", "c"], categories=["a", "c"])
-        tm.assert_index_equal(res, exp, exact=True)
-        tm.assert_numpy_array_equal(indexer, np.array([0, 3, 2], dtype=np.intp))
+        with pytest.raises(ValueError, match=msg):
+            ci.reindex(Categorical(["a", "c"]))
 
     def test_reindex_list_non_unique_unused_category(self):
+        msg = "cannot reindex on an axis with duplicate labels"
         ci = CategoricalIndex(["a", "b", "c", "a"], categories=["a", "b", "c", "d"])
-        with tm.assert_produces_warning(FutureWarning, match="non-unique"):
-            res, indexer = ci.reindex(["a", "c"])
-        exp = Index(["a", "a", "c"], dtype="object")
-        tm.assert_index_equal(res, exp, exact=True)
-        tm.assert_numpy_array_equal(indexer, np.array([0, 3, 2], dtype=np.intp))
+        with pytest.raises(ValueError, match=msg):
+            ci.reindex(["a", "c"])
 
     def test_reindex_categorical_non_unique_unused_category(self):
+        msg = "cannot reindex on an axis with duplicate labels"
         ci = CategoricalIndex(["a", "b", "c", "a"], categories=["a", "b", "c", "d"])
-        with tm.assert_produces_warning(FutureWarning, match="non-unique"):
-            res, indexer = ci.reindex(Categorical(["a", "c"]))
-        exp = CategoricalIndex(["a", "a", "c"], categories=["a", "c"])
-        tm.assert_index_equal(res, exp, exact=True)
-        tm.assert_numpy_array_equal(indexer, np.array([0, 3, 2], dtype=np.intp))
+        with pytest.raises(ValueError, match=msg):
+            ci.reindex(Categorical(["a", "c"]))
 
     def test_reindex_duplicate_target(self):
         # See GH25459

pandas/tests/resample/test_datetime_index.py

@@ -700,8 +700,7 @@ def test_asfreq_non_unique():
 
     msg = "cannot reindex on an axis with duplicate labels"
     with pytest.raises(ValueError, match=msg):
-        with tm.assert_produces_warning(FutureWarning, match="non-unique"):
-            ts.asfreq("B")
+        ts.asfreq("B")
 
 
 def test_resample_axis1():