Added tests for non-default indexes, scalar and multiple inputs, and …

…results preserve columns

doc/source/whatsnew/v0.20.0.txt

@@ -930,3 +930,5 @@ Bug Fixes
 - Bug in ``pd.melt()`` where passing a tuple value for ``value_vars`` caused a ``TypeError`` (:issue:`15348`)
 - Bug in ``.eval()`` which caused multiline evals to fail with local variables not on the first line (:issue:`15342`)
 - Bug in ``pd.read_msgpack`` which did not allow to load dataframe with an index of type ``CategoricalIndex`` (:issue:`15487`)
+
+- Bug in ``Series.asof`` which raised an error if the series contained all ``nans`` (:issue:`15713`)

pandas/core/generic.py

@@ -3972,11 +3972,11 @@ def asof(self, where, subset=None):
             where = Index(where) if is_list else Index([where])
 
         nulls = self.isnull() if is_series else self[subset].isnull().any(1)
-        if nulls.values.all():
+        if nulls.all():
             if is_series:
-                return pd.Series([np.nan])
+                return pd.Series(np.nan, index=where)
             else:
-                return pd.DataFrame([np.nan])
+                return pd.DataFrame(np.nan, index=where, columns=self.columns)
 
         locs = self.index.asof_locs(where, ~(nulls.values))
 

pandas/tests/frame/test_asof.py

@@ -13,9 +13,9 @@ class TestFrameAsof(TestData, tm.TestCase):
 
     def setUp(self):
         self.N = N = 50
-        rng = date_range('1/1/1990', periods=N, freq='53s')
+        self.rng = date_range('1/1/1990', periods=N, freq='53s')
         self.df = DataFrame({'A': np.arange(N), 'B': np.arange(N)},
-                            index=rng)
+                            index=self.rng)
 
     def test_basic(self):
 
@@ -86,3 +86,21 @@ def test_all_nans(self):
         result = DataFrame([np.nan]).asof([0])
         expected = DataFrame([np.nan])
         tm.assert_frame_equal(result, expected)
+
+        # testing non-default indexes, multiple inputs
+        dates = date_range('1/1/1990', periods=self.N * 3, freq='25s')
+        result = DataFrame(np.nan, index=self.rng, columns=['A']).asof(dates)
+        expected = DataFrame(np.nan, index=dates, columns=['A'])
+        tm.assert_frame_equal(result, expected)
+
+        # testing multiple columns
+        dates = date_range('1/1/1990', periods=self.N * 3, freq='25s')
+        result = DataFrame(np.nan, index=self.rng, columns=['A', 'B', 'C']).asof(dates)
+        expected = DataFrame(np.nan, index=dates, columns=['A', 'B', 'C'])
+        tm.assert_frame_equal(result, expected)
+
+        # testing scalar input
+        date = date_range('1/1/1990', periods=self.N * 3, freq='25s')[0]
+        result = DataFrame(np.nan, index=self.rng, columns=['A']).asof(date)
+        expected = DataFrame(np.nan, index=[date], columns=['A'])
+        tm.assert_frame_equal(result, expected)

pandas/tests/series/test_asof.py

@@ -154,3 +154,17 @@ def test_all_nans(self):
         result = Series([np.nan]).asof([0])
         expected = Series([np.nan])
         tm.assert_series_equal(result, expected)
+
+        # testing non-default indexes
+        N = 50
+        rng = date_range('1/1/1990', periods=N, freq='53s')
+
+        dates = date_range('1/1/1990', periods=N * 3, freq='25s')
+        result = Series(np.nan, index=rng).asof(dates)
+        expected = Series(np.nan, index=dates)
+        tm.assert_series_equal(result, expected)
+
+        # testing scalar input
+        date = date_range('1/1/1990', periods=N * 3, freq='25s')[0]
+        result = Series(np.nan, index=rng).asof(date)
+        self.assertTrue(result != result)