fix and test empty CFTimeIndex (#8600)

* fix empty cftimeindex repr

* switch to None

* require cftime

* fix empty cftimindex

* add tests

* none not a string in repr

* make it explicit

* explicitely test dtype of date fields

* set date_field dtype

* use repr fstring to avoid conditional

* whats new entry

* Apply suggestions from code review

Co-authored-by: Spencer Clark <spencerkclark@gmail.com>

---------

Co-authored-by: Spencer Clark <spencerkclark@gmail.com>

doc/whats-new.rst

@@ -69,7 +69,8 @@ Bug fixes
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
 - Vendor `SerializableLock` from dask and use as default lock for netcdf4 backends (:issue:`8442`, :pull:`8571`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
-
+- Add tests and fixes for empty :py:class:`CFTimeIndex`, including broken html repr (:issue:`7298`, :pull:`8600`).
+  By `Mathias Hauser <https://github.com/mathause>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/coding/cftimeindex.py

@@ -187,7 +187,7 @@ def _parsed_string_to_bounds(date_type, resolution, parsed):
 
 def get_date_field(datetimes, field):
     """Adapted from pandas.tslib.get_date_field"""
-    return np.array([getattr(date, field) for date in datetimes])
+    return np.array([getattr(date, field) for date in datetimes], dtype=np.int64)
 
 
 def _field_accessor(name, docstring=None, min_cftime_version="0.0"):
@@ -272,8 +272,8 @@ def format_attrs(index, separator=", "):
     attrs = {
         "dtype": f"'{index.dtype}'",
         "length": f"{len(index)}",
-        "calendar": f"'{index.calendar}'",
-        "freq": f"'{index.freq}'" if len(index) >= 3 else None,
+        "calendar": f"{index.calendar!r}",
+        "freq": f"{index.freq!r}",
     }
 
     attrs_str = [f"{k}={v}" for k, v in attrs.items()]
@@ -630,6 +630,10 @@ def to_datetimeindex(self, unsafe=False):
         >>> times.to_datetimeindex()
         DatetimeIndex(['2000-01-01', '2000-01-02'], dtype='datetime64[ns]', freq=None)
         """
+
+        if not self._data.size:
+            return pd.DatetimeIndex([])
+
         nptimes = cftime_to_nptime(self)
         calendar = infer_calendar_name(self)
         if calendar not in _STANDARD_CALENDARS and not unsafe:
@@ -679,6 +683,9 @@ def asi8(self):
         """Convert to integers with units of microseconds since 1970-01-01."""
         from xarray.core.resample_cftime import exact_cftime_datetime_difference
 
+        if not self._data.size:
+            return np.array([], dtype=np.int64)
+
         epoch = self.date_type(1970, 1, 1)
         return np.array(
             [
@@ -693,19 +700,29 @@ def calendar(self):
         """The calendar used by the datetimes in the index."""
         from xarray.coding.times import infer_calendar_name
 
+        if not self._data.size:
+            return None
+
         return infer_calendar_name(self)
 
     @property
     def freq(self):
         """The frequency used by the dates in the index."""
         from xarray.coding.frequencies import infer_freq
 
+        # min 3 elemtents required to determine freq
+        if self._data.size < 3:
+            return None
+
         return infer_freq(self)
 
     def _round_via_method(self, freq, method):
         """Round dates using a specified method."""
         from xarray.coding.cftime_offsets import CFTIME_TICKS, to_offset
 
+        if not self._data.size:
+            return CFTimeIndex(np.array(self))
+
         offset = to_offset(freq)
         if not isinstance(offset, CFTIME_TICKS):
             raise ValueError(f"{offset} is a non-fixed frequency")

xarray/tests/test_cftimeindex.py

@@ -238,28 +238,57 @@ def test_assert_all_valid_date_type(date_type, index):
 )
 def test_cftimeindex_field_accessors(index, field, expected):
     result = getattr(index, field)
+    expected = np.array(expected, dtype=np.int64)
     assert_array_equal(result, expected)
+    assert result.dtype == expected.dtype
+
+
+@requires_cftime
+@pytest.mark.parametrize(
+    ("field"),
+    [
+        "year",
+        "month",
+        "day",
+        "hour",
+        "minute",
+        "second",
+        "microsecond",
+        "dayofyear",
+        "dayofweek",
+        "days_in_month",
+    ],
+)
+def test_empty_cftimeindex_field_accessors(field):
+    index = CFTimeIndex([])
+    result = getattr(index, field)
+    expected = np.array([], dtype=np.int64)
+    assert_array_equal(result, expected)
+    assert result.dtype == expected.dtype
 
 
 @requires_cftime
 def test_cftimeindex_dayofyear_accessor(index):
     result = index.dayofyear
-    expected = [date.dayofyr for date in index]
+    expected = np.array([date.dayofyr for date in index], dtype=np.int64)
     assert_array_equal(result, expected)
+    assert result.dtype == expected.dtype
 
 
 @requires_cftime
 def test_cftimeindex_dayofweek_accessor(index):
     result = index.dayofweek
-    expected = [date.dayofwk for date in index]
+    expected = np.array([date.dayofwk for date in index], dtype=np.int64)
     assert_array_equal(result, expected)
+    assert result.dtype == expected.dtype
 
 
 @requires_cftime
 def test_cftimeindex_days_in_month_accessor(index):
     result = index.days_in_month
-    expected = [date.daysinmonth for date in index]
+    expected = np.array([date.daysinmonth for date in index], dtype=np.int64)
     assert_array_equal(result, expected)
+    assert result.dtype == expected.dtype
 
 
 @requires_cftime
@@ -959,6 +988,31 @@ def test_cftimeindex_calendar_property(calendar, expected):
     assert index.calendar == expected
 
 
+@requires_cftime
+def test_empty_cftimeindex_calendar_property():
+    index = CFTimeIndex([])
+    assert index.calendar is None
+
+
+@requires_cftime
+@pytest.mark.parametrize(
+    "calendar",
+    [
+        "noleap",
+        "365_day",
+        "360_day",
+        "julian",
+        "gregorian",
+        "standard",
+        "proleptic_gregorian",
+    ],
+)
+def test_cftimeindex_freq_property_none_size_lt_3(calendar):
+    for periods in range(3):
+        index = xr.cftime_range(start="2000", periods=periods, calendar=calendar)
+        assert index.freq is None
+
+
 @requires_cftime
 @pytest.mark.parametrize(
     ("calendar", "expected"),
@@ -1152,6 +1206,18 @@ def test_rounding_methods_against_datetimeindex(freq, method):
     assert result.equals(expected)
 
 
+@requires_cftime
+@pytest.mark.parametrize("method", ["floor", "ceil", "round"])
+def test_rounding_methods_empty_cftimindex(method):
+    index = CFTimeIndex([])
+    result = getattr(index, method)("2s")
+
+    expected = CFTimeIndex([])
+
+    assert result.equals(expected)
+    assert result is not index
+
+
 @requires_cftime
 @pytest.mark.parametrize("method", ["floor", "ceil", "round"])
 def test_rounding_methods_invalid_freq(method):
@@ -1230,6 +1296,14 @@ def test_asi8_distant_date():
     np.testing.assert_array_equal(result, expected)
 
 
+@requires_cftime
+def test_asi8_empty_cftimeindex():
+    index = xr.CFTimeIndex([])
+    result = index.asi8
+    expected = np.array([], dtype=np.int64)
+    np.testing.assert_array_equal(result, expected)
+
+
 @requires_cftime
 def test_infer_freq_valid_types():
     cf_indx = xr.cftime_range("2000-01-01", periods=3, freq="D")

xarray/tests/test_formatting.py

@@ -10,7 +10,7 @@
 
 import xarray as xr
 from xarray.core import formatting
-from xarray.tests import requires_dask, requires_netCDF4
+from xarray.tests import requires_cftime, requires_dask, requires_netCDF4
 
 
 class TestFormatting:
@@ -803,3 +803,18 @@ def test_format_xindexes(as_dataset: bool) -> None:
 
     actual = repr(obj.xindexes)
     assert actual == expected
+
+
+@requires_cftime
+def test_empty_cftimeindex_repr() -> None:
+    index = xr.coding.cftimeindex.CFTimeIndex([])
+
+    expected = """\
+    Indexes:
+        time     CFTimeIndex([], dtype='object', length=0, calendar=None, freq=None)"""
+    expected = dedent(expected)
+
+    da = xr.DataArray([], coords={"time": index})
+
+    actual = repr(da.indexes)
+    assert actual == expected