BUG: Datetimelike equality comparisons with Categorical

[jschendel]

Code Sample, a copy-pastable example if possible

Consider the following setup on master:

In [1]: import pandas as pd; pd.__version__
Out[1]: '0.26.0.dev0+1576.gdd0d353fb'

In [2]: dti = pd.date_range("2020", periods=3) 
   ...: dti_tz = pd.date_range("2020", periods=3, tz="UTC") 
   ...: tdi = pd.timedelta_range("0 days", periods=3) 
   ...: pi = pd.period_range("2020Q1", periods=3, freq="Q")

Equality comparisons with an equivalent Categorical are incorrect for DatetimeIndex:

In [3]: dti == pd.Categorical(dti)
Out[3]: array([False, False, False])

In [4]: dti_tz == pd.Categorical(dti_tz)
Out[4]: array([False, False, False])

Equality comparisons raise for PeriodIndex:

In [5]: pi == pd.Categorical(pi)
---------------------------------------------------------------------------
ValueError: Value must be Period, string, integer, or datetime

Looks good for TimedeltaIndex:

In [6]: tdi == pd.Categorical(tdi)
Out[6]: array([ True,  True,  True])

The incorrect behavior above is generally consistent when replacing the index with its extension array/Series equivalent, Categorical with CategoricalIndex/Series[Categorical], and == with !=.

The only exception appears to be that a couple cases work when when you have a Categorical/ CategoricalIndex on the RHS and an extension array on the LHS:

In [7]: pd.Categorical(dti) == dti.array
Out[7]: array([ True,  True,  True])

In [8]: pd.CategoricalIndex(pi) == pi.array
Out[8]: array([ True,  True,  True])

Though note that the above does not work for dti_tz.array

Problem description

Equality comparisons for datetimelike arrays/indexes are largely incorrect when comparing to equivalent categoricals. There is some tie in to #19513 but I think this specific component is pretty clear cut.

Expected Output

I'd expect all variants of == to in the examples above to return array([ True, True, True]), and all variants of != to return array([False, False, False]).

cc @jbrockmendel

[jschendel]

Worthwhile to note that equality comparisons for non-datetimelike vs. equivalent categoricals looks to be working (intervals being fixed in #30640).

Also looks like #30654 addresses the issue of PeriodIndex/PeriodArray raising and makes the behavior consistent with the DatetimeIndex/DatetimeArray examples.

[jbrockmendel]

Does it work with the Categorical on the left? i.e. will it be sufficient to make the DTA/PA ops return NotImplemented?

[jschendel]

Does it work with the Categorical on the left?

No, it works in a few additional cases that way but not for all cases, e.g.

In [1]: import pandas as pd; pd.__version__
Out[1]: '0.26.0.dev0+1637.g6d67cf9f0'

In [2]: dti_tz = pd.date_range("2020", periods=3, tz="UTC")

In [3]: pd.Categorical(dti_tz) == dti_tz
Out[3]: array([False, False, False])

In [4]: pd.Categorical(dti_tz) == dti_tz.array
Out[4]: array([False, False, False])

[tanmay1618]

Working on this

[mroeschke]

Looks like all these work as expected now. Could use tests

In [12]: In [2]: dti = pd.date_range("2020", periods=3)
    ...:    ...: dti_tz = pd.date_range("2020", periods=3, tz="UTC")
    ...:    ...: tdi = pd.timedelta_range("0 days", periods=3)
    ...:    ...: pi = pd.period_range("2020Q1", periods=3, freq="Q")

In [13]: pi == pd.Categorical(pi)
Out[13]: array([ True,  True,  True])

In [14]: dti == pd.Categorical(dti)
Out[14]: array([ True,  True,  True])

In [15]: dti_tz == pd.Categorical(dti_tz)
Out[15]: array([ True,  True,  True])

In [16]: tdi == pd.Categorical(tdi)
Out[16]: array([ True,  True,  True])

In [17]: pd.Categorical(dti) == dti.array
Out[17]: array([ True,  True,  True])

In [18]: pd.CategoricalIndex(pi) == pi.array
Out[18]: array([ True,  True,  True])

In [20]: pd.Categorical(dti_tz) == dti_tz.array
Out[20]: array([ True,  True,  True])

In [21]: pd.Categorical(dti_tz) == dti_tz
Out[21]: array([ True,  True,  True])

In [22]: pd.__version__
Out[22]: '1.1.0.dev0+1536.ga7fb88fd5'