API: DatetimeIndex / TimedeltaIndex 2d slicing should result in Index

[sinhrks]

Following slices basically result in the same Index.

import pandas as pd
pd.Index([1, 2, 3])[:, None]
# Int64Index([[1], [2], [3]], dtype='int64')
pd.MultiIndex.from_tuples([('a', 1), ('b', 2)])[:, None]
# MultiIndex(levels=[[u'a', u'b'], [1, 2]],
#            labels=[[[0], [1]], [[0], [1]]])

pd.period_range('2011-01-01', freq='M', periods=3)[:, None]
# PeriodIndex(['2011-01', '2011-02', '2011-03'], dtype='int64', freq='M')

But DatetimeIndex and TimedeltaIndex doesn't because of following line.

• https://github.com/pydata/pandas/blob/master/pandas/tseries/base.py#L119

pd.date_range('2011-01-01', freq='M', periods=3)[:, None]
# array([['2011-01-31T09:00:00.000000000+0900'],
#        ['2011-02-28T09:00:00.000000000+0900'],
#        ['2011-03-31T09:00:00.000000000+0900']], dtype='datetime64[ns]')
pd.timedelta_range('1 days', freq='D', periods=3)[:, None]
# array([[ 86400000000000],
#        [172800000000000],
#        [259200000000000]], dtype='timedelta64[ns]')

I understand these must be Index also, or any reason for this?

[jreback]

prob an oversite

but not sure we should allow 2-d results at all
it's very odd (and I don't any utility)

[sinhrks]

Yep. Raise IndexError as the same as explicit 2 indices?

pd.Index([1, 2, 3])[:, 0]
# IndexError: too many indices for array

[jreback]

sounds right

[shoyer]

I noticed this the other day. I agree with @jreback that we should raise an IndexError if the result has two or more dimensions.

[ShaharNaveh]

@jreback Since #31150 this is raising a Deprecation warning

pandas/pandas/core/indexers.py

Lines 263 to 270 in b1214af

	if np.ndim(result) > 1:
	warnings.warn(
	"Support for multi-dimensional indexing (e.g. `index[:, None]`) "
	"on an Index is deprecated and will be removed in a future "
	"version. Convert to a numpy array before indexing instead.",
	DeprecationWarning,
	stacklevel=3,
	)

---

Should it raise an IndexError instead of a deprecation warning? (as was agreed above)

[mroeschke]

Since the 2D indexing behavior is currently deprecated, we should appropriately raise once this behavior is enforced. Closing but we can reopen if we don't get the correct behavior in the future.