DISC: add accessor attributes to Index for consistency with Series

[jbrockmendel]

Broken off of #17117 for discussion, xref #8162, #17061, recent mailing list thread.

With a datetime-like column we can access year, hour, ... with self.dt.foo. With a DatetimeIndex (PeriodIndex, ...) we access these attributes directly self.foo without the .dt. I'd like to add a .dt property to the appropriate Index subclasses so that these attributes can be accessed symmetrically. i.e. instead of:

if isinstance(obj, pd.Index):
    year = obj.year
elif isinstance(obj, pd.Series):
    year = obj.dt.year

we can just use year = obj.dt.year regardless.

The implementation is three lines in core.indexes.datetimelike.DatetimeIndexOpsMixin`:

    @property
    def dt(self):
        return self

Thoughts?

[TomAugspurger]

I'd support this. It makes writing generic downstream code easier.

[jorisvandenbossche]

I would also support the idea, although I wouldn't implement it like that, because this would give you not the selection of options on tab completion (and also gives the possibility to misuse the accessor and use a wrong method eg df.index.dt.mean())

[jbrockmendel]

@jorisvandenbossche That's a good point. Shouldn't be too difficult to do implement the "right" way. It'll take some small edits to core.indexes.accessors.maybe_to_datetimelike. Implementing that will be easier if/when #17042 gets merged.

[jreback]

I think this is ok. This makes Index/Series more consistent which is a good. thing.

[jorisvandenbossche]

One thing to discuss is what it should return.

In the PR you just opened, you say:

it will return an object that is effectively index.to_series().dt

But it could also return the same as the plain index. method (which is what you originally proposed: a index.dt which would effectively return self) ? Both options are not the same:

In [72]: idx = pd.date_range("2017-01-01", periods=3)

In [74]: idx.day
Out[74]: Int64Index([1, 2, 3], dtype='int64')

In [75]: idx.to_series().dt.day
Out[75]: 
2017-01-01    1
2017-01-02    2
2017-01-03    3
Freq: D, dtype: int64

IMO it should return [74], not [75].
Although, this may defeat a bit your goal of not having to care about if something is an index or a Series? (as .dt will work on both, but still return something different)
But for that (having an index and column that really behave the same), it is maybe more worthwhile to work on #8162 instead of adding dt to the Index ?

[jreback]

this should for sure be [74]. In fact the impl is pretty trivial (and indicated at the top of the PR).

[jorisvandenbossche]

and indicated at the top of the PR).

In one of the previous ones, but not in the latest: #17204 (I have to admit I am a bit lost in all the different PRs with related content)

[jbrockmendel]

(I have to admit I am a bit lost in all the different PRs with related content)

Sorry for the inundation. I'll be slowing down shortly.

@jreback & @jorisvandenbossche
The implementation in #17204 behaves like [75]. The implementation with a property at the top of the PR behaves like [74]. I'm largely indifferent between the two, implemented the [75]-like version to address @jorisvandenbossche's comment about dir(index.dt).

The main advantage of the [75] version in #17204 is that it make index.dt actually go through the same code paths for Index and Series, so the similar behavior is not just cosmetic. I view that as a step towards e.g. #8162.