ENH: dtype-unaware (empty) objects ("any" dtype)

pandas/core/indexing.py

@@ -2116,6 +2116,8 @@ def _setitem_with_indexer_missing(self, indexer, value):
                 curr_dtype = self.obj.dtype
                 curr_dtype = getattr(curr_dtype, "numpy_dtype", curr_dtype)
                 new_dtype = maybe_promote(curr_dtype, value)[0]
+            elif self.obj.empty and is_object_dtype(self.obj.dtype):
+                new_dtype = self.obj.dtype
             else:
                 new_dtype = None
 

pandas/tests/indexing/test_loc.py

@@ -1939,7 +1939,7 @@ def test_loc_setitem_empty_series(self):
         # GH#5226
 
         # partially set with an empty object series
-        ser = Series(dtype=object)
+        ser = Series()
         ser.loc[1] = 1
         tm.assert_series_equal(ser, Series([1], index=[1]))
         ser.loc[3] = 3
@@ -1949,7 +1949,7 @@ def test_loc_setitem_empty_series_float(self):
         # GH#5226
 
         # partially set with an empty object series
-        ser = Series(dtype=object)
+        ser = Series()
         ser.loc[1] = 1.0
         tm.assert_series_equal(ser, Series([1.0], index=[1]))
         ser.loc[3] = 3.0
@@ -1959,7 +1959,7 @@ def test_loc_setitem_empty_series_str_idx(self):
         # GH#5226
 
         # partially set with an empty object series
-        ser = Series(dtype=object)
+        ser = Series()
         ser.loc["foo"] = 1
         tm.assert_series_equal(ser, Series([1], index=["foo"]))
         ser.loc["bar"] = 3

pandas/tests/series/indexing/test_set_value.py

@@ -15,7 +15,7 @@ def test_series_set_value():
     dates = [datetime(2001, 1, 1), datetime(2001, 1, 2)]
     index = DatetimeIndex(dates)
 
-    s = Series(dtype=object)
+    s = Series()
     s._set_value(dates[0], 1.0)
     s._set_value(dates[1], np.nan)
 

pandas/tests/series/indexing/test_setitem.py

@@ -468,14 +468,14 @@ class TestSetitemWithExpansion:
     def test_setitem_empty_series(self):
         # GH#10193
         key = Timestamp("2012-01-01")
-        series = Series(dtype=object)
+        series = Series()
         series[key] = 47
         expected = Series(47, [key])
         tm.assert_series_equal(series, expected)
 
     def test_setitem_empty_series_datetimeindex_preserves_freq(self):
         # GH#33573 our index should retain its freq
-        series = Series([], DatetimeIndex([], freq="D"), dtype=object)
+        series = Series([], DatetimeIndex([], freq="D"))
         key = Timestamp("2012-01-01")
         series[key] = 47
         expected = Series(47, DatetimeIndex([key], freq="D"))
@@ -1660,3 +1660,18 @@ def test_setitem_empty_mask_dont_upcast_dt64():
     ser.mask(mask, "foo", inplace=True)
     assert ser.dtype == dti.dtype  # no-op -> dont upcast
     tm.assert_series_equal(ser, orig)
+
+
+def test_setitem_on_series_dtype_object():
+    # GH#19647
+    result = Series(dtype="object")
+    result.loc["int"] = 1
+    result.loc["float"] = 2.0
+    expected = Series(data=[1, 2.0], index=["int", "float"]).astype("object")
+    tm.assert_series_equal(result, expected)
+
+    result = Series()
+    result.loc["int"] = 1
+    result.loc["float"] = 2.0
+    expected = Series(data=[1, 2.0], index=["int", "float"]).astype("float")
+    tm.assert_series_equal(result, expected)