removed object type inf from _maybe_casted_values

Author: Trevor Bye

pandas/core/frame.py

@@ -4496,10 +4496,7 @@ class    max    type
 
         def _maybe_casted_values(index, labels=None):
             values = index._values
-            if not isinstance(index, (PeriodIndex, DatetimeIndex)):
-                if values.dtype == np.object_:
-                    values = lib.maybe_convert_objects(values)
-
+            
             # if we have the labels, extract the values with a mask
             if labels is not None:
                 mask = labels == -1

pandas/tests/frame/test_set_index_preserve_dtype.py

@@ -1,13 +1,40 @@
 import pytest
-import numpy as np
 import pandas as pd
-import pandas._testing as tm
 
 
-@pytest.mark.parametrize('test_dtype', [object, 'int8', 'int16', 'int32', 'int64'])
+@pytest.mark.parametrize('test_dtype', [object, 'int64'])
 def test_dtypes(test_dtype):
     df = pd.DataFrame({'A': pd.Series([1, 2, 3], dtype=test_dtype), 'B': [1, 2, 3]})
     expected = df.dtypes.values[0].type
 
     result = df.set_index('A').index.dtype.type
     assert result == expected
+
+
+@pytest.fixture
+def mixed_series():
+    return pd.Series([1, 2, 3, 'apple', 'corn'], dtype=object)
+
+
+@pytest.fixture
+def int_series():
+    return pd.Series([100, 200, 300, 400, 500])
+
+
+def test_dtypes_between_queries(mixed_series, int_series)
+    df = pd.DataFrame({'item': mixed_series, 'cost': int_series})
+
+    orig_dtypes = df.dtypes
+    item_dtype = orig_dtypes.get('item').type
+    cost_dtype = orig_dtypes.get('cost').type
+    expected = {'item': item_dtype, 'cost': cost_dtype}
+
+    # after applying a query that would remove strings from the 'item' series with dtype: object,
+    # that series should remain as dtype: object as it becomes an index, and again as it becomes 
+    # a column again after calling reset_index()
+    dtypes_transformed = df.query('cost < 400').set_index('item').reset_index().dtypes
+    item_dtype_transformed = dtypes_transformed.get('item').type
+    cost_dtype_transformed = dtypes_transformed.get('cost').type
+    result = {'item': item_dtype_transformed, 'cost': cost_dtype_transformed}
+
+    assert result == expected