Add RangeIndex

[benbovy]

• Closes Regular (linspace) Coordinates/Index #8473
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

Work in progress (just Ready for review (copied and adapted the example from #9543 (comment)).

[benbovy]

I've made further progress on this. Some design questions (thoughts welcome!):

Create a new RangeIndex

• I'm not sure yet about the public API? Currently RangeIndex.__init__ is "private" (more flexible and easier for internals) and there are two public factories RangeIndex.arange and RangeIndex.linspace inspired from Numpy API. Creating a new dataset with a range index would look like:

import xarray as xr
from xarray.indexes import RangeIndex

index = RangeIndex.arange("x", "x", 0.0, 1.0, 0.1)
ds = xr.Dataset(coords=xr.Coordinates.from_xindex(index))

<xarray.Dataset> Size: 80B
Dimensions:  (x: 10)
Coordinates:
  * x        (x) float64 80B 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Data variables:
    *empty*
Indexes:
    x        RangeIndex

• RangeIndex doesn't support set_xindex. Do we want to support it? If yes, how would look like the input coordinate? An existing range with explicit values from which RangeIndex would try to infer a constant step value? Is that useful to have? Since the point of RangeIndex is to avoid materializing coordinate values in memory... Or a 1D coordinate with three values representing start, stop and step? Any other alternative?

Index import

Should we expose all public built-in Xarray indexes at the top level? Or only at the xarray.indexes level?

Currently the Index base class and CFTimeIndex (not an Xarray index but could eventually be refactored so) are exposed at the top level, while PandasIndex, PandasMultiIndex and RangeIndex (this PR) are only exposed at the xarray.indexes level. We might want to uniformize that.

[benbovy]

Note: this Xarray RangeIndex is designed for floating value ranges. For integer ranges it is probably best to use a PandasIndex wrapping a pandas.RangeIndex. I added a note in the docstrings here. More work on the documentation is needed but probably in a later PR addressing Xarray indexes in general.

[dcherian]

Could consider adding a closed kwarg like pd.Interval, but in a future PR of course.

[dcherian]

Can these all be cached_property?

[benbovy]

Would there be much benefit of caching those simple aliases to attributes of the underlying transform?

[max-sixty]

Suggested change

	Examples
	Note that all `start`, `stop` & `step` must be passed, which is more explicit than `np.arange` or `range`
	Examples

(optional, no strong view)

[benbovy]

Note that all start, stop & step must be passed

This isn't exactly true, but yes the API here is more explicit than np.arange and range, e.g., RangeIndex.arange(10.0) means start=10 while np.arange(10.0) means stop=10.

RangeIndex.arange(10.0) doesn't make much sense, though, considering the default value of stop=1.0. I'll see if we can get closer to np.arange using tpying.overload.

[max-sixty]

RangeIndex.arange(10.0) doesn't make much sense, though, considering the default value of stop=1.0. I'll see if we can get closer to np.arange using tpying.overload.

yeah. no objection to the more explicit approach — it's useful-but-a-bit-magic that arange / range changes the meaning of the first arg based on how many are supplied