TST: test and doc for #18515

Author: toobaz

doc/source/whatsnew/v0.22.0.txt

@@ -68,6 +68,7 @@ Other API Changes
 
 - :func:`Series.astype` and :func:`Index.astype` with an incompatible dtype will now raise a ``TypeError`` rather than a ``ValueError`` (:issue:`18231`)
 - ``Series`` construction with an ``object`` dtyped tz-aware datetime and ``dtype=object`` specified, will now return an ``object`` dtyped ``Series``, previously this would infer the datetime dtype (:issue:`18231`)
+- A :class:`Series` of ``dtype=category`` constructed from an empty ``dict`` will now have categories of ``dtype=object`` rather than ``dtype=float64``, consistently with the case in which an empty list is passed (:issue:`18515`)
 - ``NaT`` division with :class:`datetime.timedelta` will now return ``NaN`` instead of raising (:issue:`17876`)
 - All-NaN levels in a ``MultiIndex`` are now assigned ``float`` rather than ``object`` dtype, promoting consistency with ``Index`` (:issue:`17929`).
 - :class:`Timestamp` will no longer silently ignore unused or invalid ``tz`` or ``tzinfo`` keyword arguments (:issue:`17690`)

pandas/tests/series/test_constructors.py

@@ -4,6 +4,7 @@
 import pytest
 
 from datetime import datetime, timedelta
+from collections import OrderedDict
 
 from numpy import nan
 import numpy as np
@@ -79,17 +80,34 @@ def test_constructor(self):
         m = MultiIndex.from_arrays([[1, 2], [3, 4]])
         pytest.raises(NotImplementedError, Series, m)
 
-    def test_constructor_empty(self):
+    @pytest.mark.parametrize('input_class', [list, dict, OrderedDict])
+    def test_constructor_empty(self, input_class):
         empty = Series()
-        empty2 = Series([])
-
-        # the are Index() and RangeIndex() which don't compare type equal
+        empty2 = Series(input_class())
+        # these are Index() and RangeIndex() which don't compare type equal
         # but are just .equals
         assert_series_equal(empty, empty2, check_index_type=False)
 
-        empty = Series(index=lrange(10))
-        empty2 = Series(np.nan, index=lrange(10))
-        assert_series_equal(empty, empty2)
+        # With explicit dtype:
+        empty = Series(dtype='float64')
+        empty2 = Series(input_class(), dtype='float64')
+        assert_series_equal(empty, empty2, check_index_type=False)
+
+        # GH 18515 : with dtype=category:
+        empty = Series(dtype='category')
+        empty2 = Series(input_class(), dtype='category')
+        assert_series_equal(empty, empty2, check_index_type=False)
+
+        if input_class is not list:
+            # With index:
+            empty = Series(index=lrange(10))
+            empty2 = Series(input_class(), index=lrange(10))
+            assert_series_equal(empty, empty2)
+
+            # With index and dtype float64:
+            empty = Series(np.nan, index=lrange(10))
+            empty2 = Series(input_class(), index=lrange(10), dtype='float64')
+            assert_series_equal(empty, empty2)
 
     def test_constructor_series(self):
         index1 = ['d', 'b', 'a', 'c']