BUG: Index.get_loc raising incorrect error, closes #29189 (#29700)

Author: jbrockmendel

doc/source/whatsnew/v1.0.0.rst

@@ -562,7 +562,7 @@ Indexing
 - Bug in :meth:`Float64Index.astype` where ``np.inf`` was not handled properly when casting to an integer dtype (:issue:`28475`)
 - :meth:`Index.union` could fail when the left contained duplicates (:issue:`28257`)
 - :meth:`Index.get_indexer_non_unique` could fail with `TypeError` in some cases, such as when searching for ints in a string index (:issue:`28257`)
--
+- Bug in :meth:`Float64Index.get_loc` incorrectly raising ``TypeError`` instead of ``KeyError`` (:issue:`29189`)
 
 Missing
 ^^^^^^^

pandas/_libs/index.pyx

@@ -141,8 +141,12 @@ cdef class IndexEngine:
 
         if self.is_monotonic_increasing:
             values = self._get_index_values()
-            left = values.searchsorted(val, side='left')
-            right = values.searchsorted(val, side='right')
+            try:
+                left = values.searchsorted(val, side='left')
+                right = values.searchsorted(val, side='right')
+            except TypeError:
+                # e.g. GH#29189 get_loc(None) with a Float64Index
+                raise KeyError(val)
 
             diff = right - left
             if diff == 0:

pandas/tests/groupby/test_groupby.py

@@ -1942,6 +1942,16 @@ def test_groupby_only_none_group():
     tm.assert_series_equal(actual, expected)
 
 
+def test_groupby_duplicate_index():
+    # GH#29189 the groupby call here used to raise
+    ser = pd.Series([2, 5, 6, 8], index=[2.0, 4.0, 4.0, 5.0])
+    gb = ser.groupby(level=0)
+
+    result = gb.mean()
+    expected = pd.Series([2, 5.5, 8], index=[2.0, 4.0, 5.0])
+    tm.assert_series_equal(result, expected)
+
+
 @pytest.mark.parametrize("bool_agg_func", ["any", "all"])
 def test_bool_aggs_dup_column_labels(bool_agg_func):
     # 21668

pandas/tests/indexing/test_indexing.py

@@ -1209,3 +1209,16 @@ def test_1tuple_without_multiindex():
     result = ser[key]
     expected = ser[key[0]]
     tm.assert_series_equal(result, expected)
+
+
+def test_duplicate_index_mistyped_key_raises_keyerror():
+    # GH#29189 float_index.get_loc(None) should raise KeyError, not TypeError
+    ser = pd.Series([2, 5, 6, 8], index=[2.0, 4.0, 4.0, 5.0])
+    with pytest.raises(KeyError):
+        ser[None]
+
+    with pytest.raises(KeyError):
+        ser.index.get_loc(None)
+
+    with pytest.raises(KeyError):
+        ser.index._engine.get_loc(None)