Added test for gh15077 (cf. gh15115) and whatsnew note

Author: Giacomo Ferroni

doc/source/whatsnew/v0.20.0.txt

@@ -362,3 +362,4 @@ Bug Fixes
 
 
 - Bug in ``pd.read_csv()`` for the C engine where ``usecols`` were being indexed incorrectly with ``parse_dates`` (:issue:`14792`)
+- Bug in dtypes returned by comparison methods (e.g., ``lt``, ``gt``, ...) against a constant for an empty DataFrame (:issue:`15077`)

pandas/tests/frame/test_operators.py

@@ -671,6 +671,28 @@ def _test_seq(df, idx_ser, col_ser):
         exp = DataFrame({'col': [False, True, False]})
         assert_frame_equal(result, exp)
 
+    def test_return_dtypes_bool_op_costant(self):
+        # GH15077
+        df = DataFrame({'x': [1, 2, 3], 'y': [1., 2., 3.]})
+        const = 2
+
+        # not empty DataFrame
+        for op in ['eq', 'ne', 'gt', 'lt', 'ge', 'le']:
+            f = getattr(df, op)
+            for col in ['x', 'y']:
+                res = f(const)
+                c = getattr(res, col)
+                self.assertEqual(c.dtypes, bool)
+
+        # empty DataFrame
+        empty = df.iloc[:0]
+        for op in ['eq', 'ne', 'gt', 'lt', 'ge', 'le']:
+            f = getattr(empty, op)
+            for col in ['x', 'y']:
+                res = f(const)
+                c = getattr(res, col)
+                self.assertEqual(c.dtypes, bool)
+
     def test_dti_tz_convert_to_utc(self):
         base = pd.DatetimeIndex(['2011-01-01', '2011-01-02',
                                  '2011-01-03'], tz='UTC')