Convert calendar to lowercase in standard calendar checks (pydata#5180)

doc/whats-new.rst

@@ -94,6 +94,11 @@ Bug fixes
   By `Justus Magin <https://github.com/keewis>`_.
 - Decode values as signed if attribute `_Unsigned = "false"` (:issue:`4954`)
   By `Tobias Kölling <https://github.com/d70-t>`_.
+- Ensure standard calendar dates encoded with a calendar attribute with some or
+  all uppercase letters can be decoded or encoded to or from
+  ``np.datetime64[ns]`` dates with or without ``cftime`` installed
+  (:issue:`5093`, :pull:`5180`).  By `Spencer Clark
+  <https://github.com/spencerkclark>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/coding/times.py

@@ -68,6 +68,10 @@
 )
 
 
+def _is_standard_calendar(calendar):
+    return calendar.lower() in _STANDARD_CALENDARS
+
+
 def _netcdf_to_numpy_timeunit(units):
     units = units.lower()
     if not units.endswith("s"):
@@ -166,7 +170,7 @@ def _decode_datetime_with_cftime(num_dates, units, calendar):
 
 
 def _decode_datetime_with_pandas(flat_num_dates, units, calendar):
-    if calendar not in _STANDARD_CALENDARS:
+    if not _is_standard_calendar(calendar):
         raise OutOfBoundsDatetime(
             "Cannot decode times from a non-standard calendar, {!r}, using "
             "pandas.".format(calendar)
@@ -237,7 +241,7 @@ def decode_cf_datetime(num_dates, units, calendar=None, use_cftime=None):
                 dates[np.nanargmin(num_dates)].year < 1678
                 or dates[np.nanargmax(num_dates)].year >= 2262
             ):
-                if calendar in _STANDARD_CALENDARS:
+                if _is_standard_calendar(calendar):
                     warnings.warn(
                         "Unable to decode time axis into full "
                         "numpy.datetime64 objects, continuing using "
@@ -247,7 +251,7 @@ def decode_cf_datetime(num_dates, units, calendar=None, use_cftime=None):
                         stacklevel=3,
                     )
             else:
-                if calendar in _STANDARD_CALENDARS:
+                if _is_standard_calendar(calendar):
                     dates = cftime_to_nptime(dates)
     elif use_cftime:
         dates = _decode_datetime_with_cftime(flat_num_dates, units, calendar)
@@ -450,7 +454,7 @@ def encode_cf_datetime(dates, units=None, calendar=None):
 
     delta, ref_date = _unpack_netcdf_time_units(units)
     try:
-        if calendar not in _STANDARD_CALENDARS or dates.dtype.kind == "O":
+        if not _is_standard_calendar(calendar) or dates.dtype.kind == "O":
             # parse with cftime instead
             raise OutOfBoundsDatetime
         assert dates.dtype == "datetime64[ns]"

xarray/tests/test_coding_times.py

@@ -26,7 +26,7 @@
 from xarray.coding.variables import SerializationWarning
 from xarray.conventions import _update_bounds_attributes, cf_encoder
 from xarray.core.common import contains_cftime_datetimes
-from xarray.testing import assert_equal
+from xarray.testing import assert_equal, assert_identical
 
 from . import (
     arm_xfail,
@@ -1049,3 +1049,23 @@ def test__encode_datetime_with_cftime():
     expected = cftime.date2num(times, encoding_units, calendar)
     result = _encode_datetime_with_cftime(times, encoding_units, calendar)
     np.testing.assert_equal(result, expected)
+
+
+@pytest.mark.parametrize("calendar", ["gregorian", "Gregorian", "GREGORIAN"])
+def test_decode_encode_roundtrip_with_non_lowercase_letters(calendar):
+    # See GH 5093.
+    times = [0, 1]
+    units = "days since 2000-01-01"
+    attrs = {"calendar": calendar, "units": units}
+    variable = Variable(["time"], times, attrs)
+    decoded = conventions.decode_cf_variable("time", variable)
+    encoded = conventions.encode_cf_variable(decoded)
+
+    # Previously this would erroneously be an array of cftime.datetime
+    # objects.  We check here that it is decoded properly to np.datetime64.
+    assert np.issubdtype(decoded.dtype, np.datetime64)
+
+    # Use assert_identical to ensure that the calendar attribute maintained its
+    # original form throughout the roundtripping process, uppercase letters and
+    # all.
+    assert_identical(variable, encoded)