changed dtype conversion to warning to not break existing behaviour

pandas/core/series.py

@@ -319,10 +319,17 @@ def __init__(
                     data = data.copy()
             else:
                 if isinstance(data, (list, tuple)) and not data and dtype is None:
-                    # makes sure that empty Series has dtype object
-                    # while this inconsistent with numpy, it is consistent
+                    # Empty Series should have dtype object to be consistent
                     # with the behaviour of DataFrame and Index
-                    dtype = np.dtype(object)
+                    warnings.warn(
+                        "The default dtype for empty Series will be 'object' instead"
+                        " of 'float64' in the next version. Specify a dtype explicitly"
+                        " to silence this warning.",
+                        FutureWarning,
+                        stacklevel=7,
+                    )
+                    # uncomment the line below when removing the FutureWarning
+                    # dtype = np.dtype(object)
 
                 data = sanitize_array(data, index, dtype, copy, raise_cast_failure=True)
                 data = SingleBlockManager(data, index, fastpath=True)

pandas/tests/series/test_constructors.py

@@ -53,13 +53,19 @@ class TestSeriesConstructors:
         ],
     )
     def test_empty_constructor(self, constructor, check_index_type):
-        expected = Series()
-        result = constructor()
+        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
+            expected = Series()
+            result = constructor()
+
         assert len(result.index) == 0
         tm.assert_series_equal(result, expected, check_index_type=check_index_type)
 
     def test_dtype_of_empty_series(self):
-        assert Series().dtype == object
+        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
+            Series()
+
+        # no warning when dtype is specified explicitly
+        Series(dtype="object")
 
     def test_invalid_dtype(self):
         # GH15520
@@ -117,8 +123,9 @@ def test_constructor(self, datetime_series):
 
     @pytest.mark.parametrize("input_class", [list, dict, OrderedDict])
     def test_constructor_empty(self, input_class):
-        empty = Series()
-        empty2 = Series(input_class())
+        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
+            empty = Series()
+            empty2 = Series(input_class())
 
         # these are Index() and RangeIndex() which don't compare type equal
         # but are just .equals