Allow decoding of size-0 datetimes (#6882)

Author: dcherian

doc/whats-new.rst

@@ -34,6 +34,8 @@ Deprecations
 Bug fixes
 ~~~~~~~~~
 
+- Allow decoding of 0 sized datetimes(:issue:`1329`, :pull:`6882`)
+  By `Deepak Cherian <https://github.com/dcherian>`_.
 - Make sure DataArray.name is always a string when used as label for plotting.
   (:issue:`6826`, :pull:`6832`)
   By `Jimmy Westling <https://github.com/illviljan>`_.

xarray/coding/times.py

@@ -197,9 +197,12 @@ def _decode_cf_datetime_dtype(data, units, calendar, use_cftime):
 def _decode_datetime_with_cftime(num_dates, units, calendar):
     if cftime is None:
         raise ModuleNotFoundError("No module named 'cftime'")
-    return np.asarray(
-        cftime.num2date(num_dates, units, calendar, only_use_cftime_datetimes=True)
-    )
+    if num_dates.size > 0:
+        return np.asarray(
+            cftime.num2date(num_dates, units, calendar, only_use_cftime_datetimes=True)
+        )
+    else:
+        return np.array([], dtype=object)
 
 
 def _decode_datetime_with_pandas(flat_num_dates, units, calendar):
@@ -220,8 +223,10 @@ def _decode_datetime_with_pandas(flat_num_dates, units, calendar):
 
     with warnings.catch_warnings():
         warnings.filterwarnings("ignore", "invalid value encountered", RuntimeWarning)
-        pd.to_timedelta(flat_num_dates.min(), delta) + ref_date
-        pd.to_timedelta(flat_num_dates.max(), delta) + ref_date
+        if flat_num_dates.size > 0:
+            # avoid size 0 datetimes GH1329
+            pd.to_timedelta(flat_num_dates.min(), delta) + ref_date
+            pd.to_timedelta(flat_num_dates.max(), delta) + ref_date
 
     # To avoid integer overflow when converting to nanosecond units for integer
     # dtypes smaller than np.int64 cast all integer and unsigned integer dtype

xarray/tests/test_coding_times.py

@@ -1150,3 +1150,20 @@ def test_decode_cf_datetime_uint64_with_cftime_overflow_error():
     num_dates = np.uint64(1_000_000 * 86_400 * 360 * 500_000)
     with pytest.raises(OverflowError):
         decode_cf_datetime(num_dates, units, calendar)
+
+
+@pytest.mark.parametrize("use_cftime", [True, False])
+def test_decode_0size_datetime(use_cftime):
+    # GH1329
+    if use_cftime and not has_cftime:
+        pytest.skip()
+
+    dtype = object if use_cftime else "M8[ns]"
+    expected = np.array([], dtype=dtype)
+    actual = decode_cf_datetime(
+        np.zeros(shape=0, dtype=np.int64),
+        units="days since 1970-01-01 00:00:00",
+        calendar="proleptic_gregorian",
+        use_cftime=use_cftime,
+    )
+    np.testing.assert_equal(expected, actual)