ENH: Resolutions for month/qtr/year

[jbrockmendel]

This does three things

1. Use Resolution objects instead of strings in the places in core.indexes. This makes whats going on much clearer.
2. To make 1) possible, extend Resolution to include month, quarter, and year. This means that pd.period_range("2017", periods=3, freq="A").resolution is now "year" instead of "day", which is more accurate.
3. with year/quarter/month added, Resolution and FreqGroup will be re-labellings of each other, so we can de-duplicate them. They will end up living in tslibs.dtypes.

[pep8speaks]

Hello @jbrockmendel! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-06-09 21:06:34 UTC

[jorisvandenbossche]

Some questions:

• To be able to de-duplicate with FreqGroup, wouldn't you also need to add weekly and business-day resolution?
• Since resolution is used for string datetime indexing, how does this change affect that (or not) ?

[jbrockmendel]

To be able to de-duplicate with FreqGroup, wouldn't you also need to add weekly and business-day resolution?

Yep, oversight on my part. Weekly we could reasonably add to Resolution, but Business makes less sense. De-duplication may not be viable after all.

Since resolution is used for string datetime indexing, how does this change affect that (or not) ?

It does not affect the behavior. Adding the month/quarter/year resos is what allows us to use Resolution objects in the relevant indexing code without changing any behavior.

[jreback]

ok if/elif and needs a rebase

[jbrockmendel]

rebased+green

[jreback]

thanks!