Merge pull request #11155 from kawochen/BUG-FIX-11086

BUG/API: GH11086 where freq is not inferred if both freq is None

Author: jreback

doc/source/whatsnew/v0.17.0.txt

@@ -916,7 +916,7 @@ Other API Changes
 - When constructing ``DataFrame`` with an array of ``complex64`` dtype previously meant the corresponding column
   was automatically promoted to the ``complex128`` dtype. Pandas will now preserve the itemsize of the input for complex data (:issue:`10952`)
 - some numeric reduction operators would return ``ValueError``, rather than ``TypeError`` on object types that includes strings and numbers (:issue:`11131`)
-
+- ``DatetimeIndex.union`` does not infer ``freq`` if ``self`` and the input have ``None`` as ``freq`` (:issue:`11086`)
 - ``NaT``'s methods now either raise ``ValueError``, or return ``np.nan`` or ``NaT`` (:issue:`9513`)
 
   ===============================     ===============================================================

pandas/io/pytables.py

@@ -51,7 +51,7 @@
 _default_encoding = 'UTF-8'
 
 def _ensure_decoded(s):
-    """ if we have bytes, decode them to unicde """
+    """ if we have bytes, decode them to unicode """
     if isinstance(s, np.bytes_):
         s = s.decode('UTF-8')
     return s

pandas/tseries/index.py

@@ -895,7 +895,8 @@ def union(self, other):
             result = Index.union(this, other)
             if isinstance(result, DatetimeIndex):
                 result.tz = this.tz
-                if result.freq is None:
+                if (result.freq is None and
+                        (this.freq is not None or other.freq is not None)):
                     result.offset = to_offset(result.inferred_freq)
             return result
 

pandas/tseries/tests/test_timeseries.py

@@ -2551,6 +2551,15 @@ def test_intersection_bug_1708(self):
         result = index_1 & index_2
         self.assertEqual(len(result), 0)
 
+    def test_union_freq_both_none(self):
+        #GH11086
+        expected = bdate_range('20150101', periods=10)
+        expected.freq = None
+
+        result = expected.union(expected)
+        tm.assert_index_equal(result, expected)
+        self.assertIsNone(result.freq)
+
     # GH 10699
     def test_datetime64_with_DateOffset(self):
         for klass, assert_func in zip([Series, DatetimeIndex],