more upstream-dev cftime failures

[dcherian]

https://dev.azure.com/xarray/xarray/_build/results?buildId=2116&view=logs&jobId=2280efed-fda1-53bd-9213-1fa8ec9b4fa8&j=2280efed-fda1-53bd-9213-1fa8ec9b4fa8&t=175181ee-1928-5a6b-f537-168f7a8b7c2d

46 failed tests but they all seem to result from the same TypeError

=================================== FAILURES ===================================
______________ test_sel_date_scalar_nearest[365_day-sel_kwargs0] _______________

da = <xarray.DataArray (time: 4)>
             0002-02-01 00:00:00],
            dtype='object')
sel_kwargs = {'method': 'nearest'}

    @requires_cftime
    @pytest.mark.parametrize(
        "sel_kwargs",
        [{"method": "nearest"}, {"method": "nearest", "tolerance": timedelta(days=70)}],
    )
    def test_sel_date_scalar_nearest(da, date_type, index, sel_kwargs):
        expected = xr.DataArray(2).assign_coords(time=index[1])
>       result = da.sel(time=date_type(1, 4, 1), **sel_kwargs)

xarray/tests/test_cftimeindex.py:460: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
xarray/core/dataarray.py:1056: in sel
    ds = self._to_temp_dataset().sel(
xarray/core/dataset.py:2056: in sel
    pos_indexers, new_indexes = remap_label_indexers(
xarray/core/coordinates.py:391: in remap_label_indexers
    pos_indexers, new_indexes = indexing.remap_label_indexers(
xarray/core/indexing.py:270: in remap_label_indexers
    idxr, new_idx = convert_label_indexer(index, label, dim, method, tolerance)
xarray/core/indexing.py:189: in convert_label_indexer
    indexer = index.get_loc(
xarray/coding/cftimeindex.py:334: in get_loc
    return pd.Index.get_loc(self, key, method=method, tolerance=tolerance)
/usr/share/miniconda/envs/xarray-tests/lib/python3.8/site-packages/pandas/core/indexes/base.py:2899: in get_loc
    indexer = self.get_indexer([key], method=method, tolerance=tolerance)
/usr/share/miniconda/envs/xarray-tests/lib/python3.8/site-packages/pandas/core/indexes/base.py:2992: in get_indexer
    indexer = self._get_nearest_indexer(target, limit, tolerance)
/usr/share/miniconda/envs/xarray-tests/lib/python3.8/site-packages/pandas/core/indexes/base.py:3076: in _get_nearest_indexer
    left_distances = np.abs(self[left_indexer] - target)
xarray/coding/cftimeindex.py:444: in __sub__
    return CFTimeIndex(np.array(self) - other)
xarray/coding/cftimeindex.py:248: in __new__
    assert_all_valid_date_type(data)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

data = TimedeltaIndex(['-59 days'], dtype='timedelta64[ns]', freq=None)

    def assert_all_valid_date_type(data):
        import cftime
    
        if len(data) > 0:
            sample = data[0]
            date_type = type(sample)
            if not isinstance(sample, cftime.datetime):
>               raise TypeError(
                    "CFTimeIndex requires cftime.datetime "
                    "objects. Got object of {}.".format(date_type)
                )
E               TypeError: CFTimeIndex requires cftime.datetime objects. Got object of <class 'pandas._libs.tslibs.timedeltas.Timedelta'>.

xarray/coding/cftimeindex.py:206: TypeError

[spencerkclark]

Part of the hazard of using a pd.Index subclass I suppose...

It looks like pandas-dev/pandas#31511 was the cause of the issue:

$ git bisect good
64336ff8414f8977ff94adb9a5bc000a3a4ef454 is the first bad commit
commit 64336ff8414f8977ff94adb9a5bc000a3a4ef454
Author: Kevin Anderson <57452607+kanderso-nrel@users.noreply.github.com>
Date:   Sun Feb 2 20:48:28 2020 -0700

    BUG: fix reindexing with a tz-aware index and method='nearest' (#31511)

 doc/source/whatsnew/v1.1.0.rst               |  2 +-
 pandas/core/indexes/base.py                  |  5 ++---
 pandas/tests/frame/indexing/test_indexing.py | 10 ++++++++++
 3 files changed, 13 insertions(+), 4 deletions(-)

A way to fix this upstream would be to make sure that target has the same type as the index (here it is a generic pd.Index instead of a CFTimeIndex), but I'm not sure how hard that would be (or if it makes sense in all cases):
https://github.com/pandas-dev/pandas/blob/a2a35a86c4064d297c8b48ecfea80e9f05e27712/pandas/core/indexes/base.py#L3080

I think it's possible we could work around this in xarray. It comes down to properly recognizing what to do when you subtract a generic pd.Index of cftime.datetime objects from a CFTimeIndex. Previously this code in pandas operated strictly using NumPy arrays, so there was no casting issue when doing the subtraction.

[dcherian]

Thanks for narrowing it down @spencerkclark . Let's see what @TomAugspurger thinks about an xarray workaround vs a pandas fix.

[TomAugspurger]

FWIW, I think @jbrockmendel is still progressing on an "extension index" interface where you could have a custom dtype / Index subclass that would be properly supported. Long-term, that's the best solution.

Short-term, I'm less sure what's best.

[jbrockmendel]

xarray/coding/cftimeindex.py:444: in __sub__
    return CFTimeIndex(np.array(self) - other)

any idea what other is here? looks like it might be a DatetimeIndex

[spencerkclark]

FWIW, I think @jbrockmendel is still progressing on an "extension index" interface where you could have a custom dtype / Index subclass that would be properly supported. Long-term, that's the best solution.

Nice -- I look forward to being able to try that out!

any idea what other is here? looks like it might be a DatetimeIndex

@jbrockmendel agreed that it's unclear -- we probably should have written the code for that method in a clearer way. I think it's mainly used for subtracting a single datetime.timedelta or NumPy array of datetime.timedelta objects from a CFTimeIndex. In both of those cases we would expect the result to remain a CFTimeIndex.

cftime.datetime objects often represent dates from non-standard calendars (e.g. calendars with no leap year, or calendars where all months have 30 days), so in general they are not compatible with the dates used in a DatetimeIndex. Subtracting like-calendar cftime.datetime objects is fair game though, and we'd like that to produce timedeltas.

[keewis]

when the tests fail, other is a pandas.Index containing whatever has been used to index. The subtraction results in a normal TimedeltaIndex which is then passed to the CFTimeIndex:

np.array(self) = array([cftime.DatetimeNoLeap(0001-02-01 00:00:00)], dtype=object)
other = Index([0001-05-01 00:00:00], dtype='object')
other[0] = cftime.DatetimeNoLeap(0001-05-01 00:00:00)
np.array(self) - other = TimedeltaIndex(['-89 days'], dtype='timedelta64[ns]', freq=None)

I think the issue here is that other is a pandas.Index instead of a CFTimeIndex.

[spencerkclark]

I think the issue here is that other is a pandas.Index instead of a CFTimeIndex.

Yes, I noted that in my original post (sorry if that wasn't clear).

[keewis]

no, that's my bad, it is pretty clear but I seem to have skipped over it

[spencerkclark]

Another kind of failure came up in the context of indexing a Series with a cftime.datetime object:

Example failure

____________________________ test_indexing_in_series_getitem[365_day] _____________________________

series = 0001-01-01 00:00:00    1
0001-02-01 00:00:00    2
0002-01-01 00:00:00    3
0002-02-01 00:00:00    4
dtype: int64
index = CFTimeIndex([0001-01-01 00:00:00, 0001-02-01 00:00:00, 0002-01-01 00:00:00,
             0002-02-01 00:00:00],
            dtype='object')
scalar_args = [cftime.DatetimeNoLeap(0001-01-01 00:00:00)]
range_args = ['0001', slice('0001-01-01', '0001-12-30', None), slice(None, '0001-12-30', None), slice(cftime.DatetimeNoLeap(0001-01...:00), cftime.DatetimeNoLeap(0001-12-30 00:00:00), None), slice(None, cftime.DatetimeNoLeap(0001-12-30 00:00:00), None)]

    @requires_cftime
    def test_indexing_in_series_getitem(series, index, scalar_args, range_args):
        for arg in scalar_args:
>           assert series[arg] == 1

test_cftimeindex.py:597:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../../../pandas/pandas/core/series.py:884: in __getitem__
    return self._get_with(key)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = 0001-01-01 00:00:00    1
0001-02-01 00:00:00    2
0002-01-01 00:00:00    3
0002-02-01 00:00:00    4
dtype: int64
key = cftime.DatetimeNoLeap(0001-01-01 00:00:00)

    def _get_with(self, key):
        # other: fancy integer or otherwise
        if isinstance(key, slice):
            # _convert_slice_indexer to determing if this slice is positional
            #  or label based, and if the latter, convert to positional
            slobj = self.index._convert_slice_indexer(key, kind="getitem")
            return self._slice(slobj)
        elif isinstance(key, ABCDataFrame):
            raise TypeError(
                "Indexing a Series with DataFrame is not "
                "supported, use the appropriate DataFrame column"
            )
        elif isinstance(key, tuple):
            try:
                return self._get_values_tuple(key)
            except ValueError:
                # if we don't have a MultiIndex, we may still be able to handle
                #  a 1-tuple.  see test_1tuple_without_multiindex
                if len(key) == 1:
                    key = key[0]
                    if isinstance(key, slice):
                        return self._get_values(key)
                raise

        if not isinstance(key, (list, np.ndarray, ExtensionArray, Series, Index)):
>           key = list(key)
E           TypeError: 'cftime._cftime.DatetimeNoLeap' object is not iterable

../../../pandas/pandas/core/series.py:911: TypeError

Admittedly I think most people probably use a CFTimeIndex within xarray data structures, but it would be nice to maintain some ability to use it in pandas data structures too. This issue stems from the changes made in pandas-dev/pandas#31399. I think the problem is that pandas.core.dtypes.common.is_scalar returns False for a cftime.datetime object:

In [1]: import cftime

In [2]: import pandas

In [3]: pandas.core.dtypes.common.is_scalar(cftime.DatetimeNoLeap(2000, 1, 1))
Out[3]: False

Could there be a simple upstream fix for this?

[jbrockmendel]

Could there be a simple upstream fix for this?

Yah, pandas recently added a check for is_scalar(key) in Series.__getitem__ to try to avoid some unnecessary lookups. That will need to be changed to accommodate unexpected scalars.

[spencerkclark]

@jbrockmendel @TomAugspurger it turns out that fixing indexing with the "nearest" method without overriding private methods of pandas.Index is harder than I expected within xarray alone (see #3764 (comment) for more details). Do you think an upstream fix would be acceptable here? My understanding is that the issue that prompted the change in pandas only pertained to DatetimeIndexes (or perhaps maybe also PeriodIndexes in the future); would it be possible to limit the scope of the updates in pandas-dev/pandas#31511 to just those?

[jbrockmendel]

Do you think an upstream fix would be acceptable here?

Definitely. Is it still the case that the identified problems would all be solved by having is_scalar recognize cftime.datetime?

[spencerkclark]

Thanks @jbrockmendel -- I think there are two separate issues:

• Indexing with __getitem__ in a Series would be solved by the is_scalar fix.
• Indexing with the "nearest" method would require reverting BUG: fix reindexing with a tz-aware index and method='nearest' pandas-dev/pandas#31511 for non-Datetime-or-Period-Indexes.