issues: 430460404

The main issue is that we were resampling the same time indexes across a large range of frequencies, in some cases producing very long results, e.g. resampling an index that spans 27 years to a frequency of 12 hours.

This modifies the primary test so that it constructs time indexes whose ranges are based on the frequencies we resample to. Now in total the tests in test_cftimeindex_resample.py take around 6 seconds.

@jwenfai I did some coverage analysis offline, and these tests produce the same coverage that we had before (I found it necessary to be sure to test cases where the reference index had either a shorter or longer frequency than the resample frequency). Do you think what I have here is sufficient? I think we could potentially shorten things even more, but I'm not sure if it's worth the effort.

• [x] Closes #2874

See below for the new profiling results; now the longest cftime tests are no longer associated with resample.

$ pytest -k cftime --durations=50 ... 0.18s call xarray/tests/test_backends.py::TestScipyInMemoryData::test_roundtrip_cftime_datetime_data 0.11s call xarray/tests/test_backends.py::TestScipyFilePath::test_roundtrip_cftime_datetime_data 0.10s call xarray/tests/test_backends.py::TestNetCDF4Data::test_roundtrip_cftime_datetime_data 0.09s call xarray/tests/test_backends.py::TestNetCDF4ClassicViaNetCDF4Data::test_roundtrip_cftime_datetime_data 0.09s call xarray/tests/test_backends.py::TestNetCDF4ViaDaskData::test_roundtrip_cftime_datetime_data 0.08s teardown xarray/tests/test_cftime_offsets.py::test_add_year_end_onOffset[julian-(2, 12)-()-<YearEnd: n=-1, month=12>-(1, 12)-()] 0.06s call xarray/tests/test_backends.py::TestNetCDF3ViaNetCDF4Data::test_roundtrip_cftime_datetime_data 0.06s call xarray/tests/test_backends.py::TestGenericNetCDFData::test_roundtrip_cftime_datetime_data 0.05s call xarray/tests/test_backends.py::TestScipyFileObject::test_roundtrip_cftime_datetime_data 0.04s call xarray/tests/test_conventions.py::TestCFEncodedDataStore::test_roundtrip_cftime_datetime_data 0.03s call xarray/tests/test_dataset.py::test_differentiate_cftime[True] 0.03s call xarray/tests/test_dataset.py::test_trapz_datetime[cftime-True] 0.02s call xarray/tests/test_coding_times.py::test_contains_cftime_datetimes_dask_3d[standard] 0.02s call xarray/tests/test_backends.py::test_use_cftime_standard_calendar_default_out_of_range[2500-gregorian] 0.02s call xarray/tests/test_dataset.py::test_differentiate_cftime[False] 0.02s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-right-None-4A-MAY] 0.02s call xarray/tests/test_backends.py::test_use_cftime_standard_calendar_default_in_range[gregorian] 0.02s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-None-right-11Q-JUN] 0.02s call xarray/tests/test_backends.py::test_use_cftime_standard_calendar_default_out_of_range[2500-proleptic_gregorian] 0.02s call xarray/tests/test_backends.py::test_use_cftime_true[1500-gregorian] 0.02s call xarray/tests/test_backends.py::test_use_cftime_true[2500-proleptic_gregorian] 0.01s call xarray/tests/test_backends.py::test_use_cftime_true[2000-gregorian] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-None-right-4A-MAY] 0.01s call xarray/tests/test_backends.py::test_use_cftime_standard_calendar_default_out_of_range[2500-standard] 0.01s call xarray/tests/test_backends.py::test_use_cftime_true[1500-julian] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-left-right-4A-MAY] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-right-None-11Q-JUN] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-None-right-4A-MAY] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-left-None-7M] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-left-None-4A-MAY] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-left-right-4A-MAY] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-None-None-11Q-JUN] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-right-right-4A-MAY] 0.01s call xarray/tests/test_backends.py::test_use_cftime_standard_calendar_default_out_of_range[1500-proleptic_gregorian] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-left-None-11Q-JUN] 0.01s call xarray/tests/test_backends.py::test_use_cftime_true[2500-julian] 0.01s call xarray/tests/test_backends.py::test_use_cftime_standard_calendar_default_out_of_range[1500-gregorian] 0.01s call xarray/tests/test_backends.py::test_use_cftime_true[1500-proleptic_gregorian] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-left-right-11Q-JUN] 0.01s call xarray/tests/test_backends.py::test_use_cftime_true[2000-standard] 0.01s call xarray/tests/test_backends.py::test_use_cftime_true[2500-standard] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-None-None-4A-MAY] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-right-right-11Q-JUN] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-right-right-7M] 0.01s call xarray/tests/test_backends.py::test_use_cftime_true[2500-gregorian] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-24-left-right-7M] 0.01s call xarray/tests/test_backends.py::test_use_cftime_true[2000-proleptic_gregorian] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-right-None-7M] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-None-right-7M] 0.01s call xarray/tests/test_cftimeindex_resample.py::test_resample[longer_da_freq-31-left-None-11Q-JUN]