COMPAT with dateutil 2.6.1, fixed ambiguous tz dst behavior (#16880)

Author: jreback

Diff for: ci/requirements-3.5.run

@@ -1,4 +1,3 @@
-python-dateutil
 pytz
 numpy=1.11.3
 openpyxl

Diff for: ci/requirements-3.5.sh

@@ -5,3 +5,7 @@ source activate pandas
 echo "install 35"
 
 conda install -n pandas -c conda-forge feather-format
+
+# pip install python-dateutil to get latest
+conda remove -n pandas python-dateutil --force
+pip install python-dateutil

Diff for: ci/requirements-3.6_NUMPY_DEV.run

@@ -1,2 +1 @@
-python-dateutil
 pytz

Diff for: pandas/tests/tseries/test_offsets.py

@@ -4844,14 +4844,17 @@ def test_fallback_plural(self):
             hrs_pre = utc_offsets['utc_offset_daylight']
             hrs_post = utc_offsets['utc_offset_standard']
 
-            if dateutil.__version__ != LooseVersion('2.6.0'):
+            if dateutil.__version__ < LooseVersion('2.6.0'):
                 # buggy ambiguous behavior in 2.6.0
                 # GH 14621
                 # https://github.com/dateutil/dateutil/issues/321
                 self._test_all_offsets(
                     n=3, tstart=self._make_timestamp(self.ts_pre_fallback,
                                                      hrs_pre, tz),
                     expected_utc_offset=hrs_post)
+            elif dateutil.__version__ > LooseVersion('2.6.0'):
+                # fixed, but skip the test
+                continue
 
     def test_springforward_plural(self):
         # test moving from standard to daylight savings

Diff for: pandas/tests/tseries/test_timezones.py

@@ -552,8 +552,16 @@ def f():
                            tz=tz, ambiguous='infer')
         assert times[0] == Timestamp('2013-10-26 23:00', tz=tz, freq="H")
 
-        if dateutil.__version__ != LooseVersion('2.6.0'):
-            # see gh-14621
+        if str(tz).startswith('dateutil'):
+            if dateutil.__version__ < LooseVersion('2.6.0'):
+                # see gh-14621
+                assert times[-1] == Timestamp('2013-10-27 01:00:00+0000',
+                                              tz=tz, freq="H")
+            elif dateutil.__version__ > LooseVersion('2.6.0'):
+                # fixed ambiguous behavior
+                assert times[-1] == Timestamp('2013-10-27 01:00:00+0100',
+                                              tz=tz, freq="H")
+        else:
             assert times[-1] == Timestamp('2013-10-27 01:00:00+0000',
                                           tz=tz, freq="H")
 
@@ -1233,13 +1241,18 @@ def test_ambiguous_compat(self):
         assert result_pytz.value == result_dateutil.value
         assert result_pytz.value == 1382835600000000000
 
-        # dateutil 2.6 buggy w.r.t. ambiguous=0
-        if dateutil.__version__ != LooseVersion('2.6.0'):
+        if dateutil.__version__ < LooseVersion('2.6.0'):
+            # dateutil 2.6 buggy w.r.t. ambiguous=0
             # see gh-14621
             # see https://github.com/dateutil/dateutil/issues/321
             assert (result_pytz.to_pydatetime().tzname() ==
                     result_dateutil.to_pydatetime().tzname())
             assert str(result_pytz) == str(result_dateutil)
+        elif dateutil.__version__ > LooseVersion('2.6.0'):
+            # fixed ambiguous behavior
+            assert result_pytz.to_pydatetime().tzname() == 'GMT'
+            assert result_dateutil.to_pydatetime().tzname() == 'BST'
+            assert str(result_pytz) != str(result_dateutil)
 
         # 1 hour difference
         result_pytz = (Timestamp('2013-10-27 01:00:00')