Implement sort=True for Index setops

[TomAugspurger]

#25063 made the 0.24.x sort=True behavior be enabled by sort=None. This allows us to use sort=True to mean "always sort".

So rather than raising

In [1]: import pandas as pd

In [2]: idx = pd.Index([1, 0])

In [3]: idx.union(idx, sort=True)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-92d6bafc02d6> in <module>
----> 1 idx.union(idx, sort=True)

~/sandbox/pandas/pandas/core/indexes/base.py in union(self, other, sort)
   2319         Index(['a', 'b', 'c', 'd', 1, 2, 3, 4], dtype='object')
   2320         """
-> 2321         self._validate_sort_keyword(sort)
   2322         self._assert_can_do_setop(other)
   2323

~/sandbox/pandas/pandas/core/indexes/base.py in _validate_sort_keyword(self, sort)
   2263         if sort not in [None, False]:
   2264             raise ValueError(
-> 2265                 "The 'sort' keyword only takes the values of "
   2266                 f"None or False; {sort} was passed."
   2267             )

ValueError: The 'sort' keyword only takes the values of None or False; True was passed.

We would instead return

Int64Index([0, 1], dtype='int64')

This is a (hopefully) exhaustive list of special cases not sorted when sort=None.

• union:
  • equal
  • length-0
  • incomparable
• intersection
  • equal
  • incomparable
• difference
  • incomparable
• symmetric_difference
  • incomparable

[TomAugspurger]

@jorisvandenbossche OK with moving off the 1.0 milestone?

[TomAugspurger]

Pushing off 1.0.

[jbrockmendel]

@TomAugspurger @jorisvandenbossche @jreback this issue was on the agenda for today's call but no one present knew what the reasoning was for having sort=None instead of sort=True in the first place. There are a bunch of comments in the tests to the effect of "# TODO: decide what sort=True means"

IIUC sort=True will differ from sort=None only in the handful of cases that currently go through a fastpath and avoid sorting like the [1, 0] example in the OP?