0.24.0 DateTimeIndex: union() got an unexpected keyword argument 'sort'

[bilalkamoon]

Code Sample, a copy-pastable example if possible

import pandas as pd
index1 = pd.DataFrame(index=pd.to_datetime(['19900101'])).index
index2 = index1.copy()
index1.union(index2, sort=False)
TypeError: union() got an unexpected keyword argument 'sort'

Problem description

The 'sort' argument works with 'difference' and 'intersection' but fails on 'union' for DateTimeIndex.

[TomAugspurger]

xref #24966 (that's for intersection, just TimedeltaIndex).

cc @reidy-p.

[reidy-p]

@TomAugspurger thanks.

This is just not implemented yet for DatetimeIndex. I have started work on implementing this for DatetimeIndex.union and it is quite straightforward except in the case where this._fast_union(other) is called:

pandas/pandas/core/indexes/datetimes.py

Lines 479 to 501 in ece58cb

	if len(other) == 0 or self.equals(other) or len(self) == 0:
	return super(DatetimeIndex, self).union(other)
	if not isinstance(other, DatetimeIndex):
	try:
	other = DatetimeIndex(other)
	except TypeError:
	pass
	this, other = self._maybe_utc_convert(other)
	if this._can_fast_union(other):
	return this._fast_union(other)
	else:
	result = Index.union(this, other)
	if isinstance(result, DatetimeIndex):
	# TODO: we shouldn't be setting attributes like this;
	# in all the tests this equality already holds
	result._data._dtype = this.dtype
	if (result.freq is None and
	(this.freq is not None or other.freq is not None)):
	result.freq = to_offset(result.inferred_freq)
	return result

I'm still trying to figure out whether we can do the sorting inside of _fast_union.