Reduce length of cftime resample tests

[spencerkclark]

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.

• Closes xarray/tests/test_cftimeindex_resample.py::test_resampler is way too slow #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]

[shoyer]

Looks good to me. Thank you for looking into this promptly!

Let's wait for @jwenfai to review before merging.

[jwenfai]

Thanks for taking on the task of shortening test times! If the coverage is the same, I think the rewritten tests should be good.

Just two things I feel I should mention:

• Testing even and odd multiples for resampling frequencies for a frequency class (e.g., '11MS' and '12M' for monthlies)
  I don't quite remember what the issue was but there were tests that passed for even/odd resampling frequencies but fail for the other. Perhaps the tests could be rewritten to (1) switch da_freq and freq and (2) use odd frequencies for constructing the DataArray so that when * 2 and \\ 2 operations are performed, even and odd resampling frequencies are obtained. There were some issues with resampling to 12H so maybe use a frequency that multiplies or divides to 12H.
• Testing resampling from one freq type to another (e.g., from '3Q' to '11MS')
  Again I don't remember the details other than that a problem exists/existed. Perhaps something to do with _get_range_edges since that's the part of the code where the original index of a DataArray interacts with the resampling frequency.

Both of the problems I mentioned might have been from the earliest iteration of CFTimeIndex resampling so they might have no relevance now.

[spencerkclark]

Thanks for having a look @jwenfai! I updated my PR following your suggestions; this added some more tests so now the total time is around 9 seconds.

I opted for listing the pairs of initial and resample frequencies explicitly rather than use a function to dynamically set one based on the other (I feel like it makes the tests easier to understand; also writing a function that converts one type of frequency to another similar-length-but-different-type frequency is messy).

[jwenfai]

Wow, that's quick. The updated tests look fine to me so go ahead and merge it.

[spencerkclark]

I'm going to go ahead and merge this; we can revisit things again if we want to continue to speed these tests up.