BUG: Index.get_loc raising incorrect error, closes pandas-dev#29189

Author: jbrockmendel

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

@@ -1951,3 +1951,13 @@ def test_groupby_only_none_group():
     expected = pd.Series([np.nan], name="x")
 
     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., 4., 5.])
+    tm.assert_series_equal(result, expected)

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)