convert array of netCDF4.datetime objects to numpy.datetime64 array t…

…o support virtual_variable indexing, pydata#121

test/test_conventions.py

@@ -125,12 +125,15 @@ def test_decoded_cf_datetime_array(self):
         self.assertEqual(actual.dtype, np.dtype('datetime64[ns]'))
         self.assertArrayEqual(actual, expected)
 
-        num_dates = [722000, 720000.5]
-        units = 'days since 0001-01-01 0:0:0'
         calendar = 'noleap'
-        actual = conventions.DecodedCFDatetimeArray(num_dates, units, calendar)
-        expected = nc4.num2date(num_dates, units, calendar)
-        self.assertEqual(actual.dtype, np.dtype('O'))
+        units = 'days since 0001-01-01'
+        times = pd.date_range('2001-01-01-00', end='2001-07-31-23', freq='H')
+        noleap_time = nc4.date2num(times.to_pydatetime(), units,
+                                   calendar=calendar)
+        expected = times.values
+        actual = conventions.decode_cf_datetime(noleap_time, units,
+                                                calendar=calendar)
+        self.assertEqual(actual.dtype, np.dtype('M8[ns]'))
         self.assertArrayEqual(actual, expected)
 
     @requires_netCDF4

xray/conventions.py

@@ -88,7 +88,8 @@ def nan_safe_num2date(num):
     if ((calendar not in _STANDARD_CALENDARS
             or min_date.year < 1678 or max_date.year >= 2262)
             and min_date is not pd.NaT):
-        dates = nc4.num2date(num_dates, units, calendar)
+        ncdates = nc4.num2date(num_dates, units, calendar)
+        dates = nctime_to_nptime(ncdates)
     else:
         # we can safely use np.datetime64 with nanosecond precision (pandas
         # likes ns precision so it can directly make DatetimeIndex objects)
@@ -144,6 +145,15 @@ def guess_time_units(dates):
     return '%s since %s' % (time_unit, dates[0])
 
 
+def nctime_to_nptime(times):
+    """Given an array of netCDF4.datetime objects, return an array of
+    numpy.datetime64 objects"""
+    new = np.empty(len(times), dtype='M8[ns]')
+    for i, t in enumerate(times):
+        new[i] = np.datetime64(datetime(*t.timetuple()[:6]))
+    return new
+
+
 def encode_cf_datetime(dates, units=None, calendar=None):
     """Given an array of datetime objects, returns the tuple `(num, units,
     calendar)` suitable for a CF complient time variable.
@@ -246,13 +256,7 @@ def __init__(self, array, units, calendar=None):
 
     @property
     def dtype(self):
-        if self.calendar is None or self.calendar in _STANDARD_CALENDARS:
-            # TODO: return the proper dtype (object) for a standard calendar
-            # that can't be expressed in ns precision. Perhaps we could guess
-            # this from the units?
-            return np.dtype('datetime64[ns]')
-        else:
-            return np.dtype('O')
+        return np.dtype('datetime64[ns]')
 
     def __getitem__(self, key):
         return decode_cf_datetime(self.array, units=self.units,