Make passing a DataArray for the xarray.concat dim argument equivalent to passing a pandas Index

[ceridwen]

Extending from #839, if I'm concatenating some DataArrays using concat,

print(xarray.concat(data, xarray.DataArray(['foo1', 'foo2', 'foo3', 'foo4', 'foo5'], name='stat'))

I get an unnamed dimension without coordinates.

<xarray.DataArray (dim_0: 5, index: 2)>
array([[ 24.841064,   0.750451],
       [ 24.841064,   0.750451],
       [ 19.062874,   0.796722],
       [ 14.9631  ,   0.354273],
       [ 14.9631  ,   0.354273]])
Coordinates:
  * index    (index) object 'Intercept' 'Lvl'
         (dim_0) <U3 'foo1' foo2' 'foo3' 'foo4' 'foo5'
Dimensions without coordinates: dim_0

Using a pandas.Index,

print(xarray.concat(data, pandas.Index(['foo1', 'foo2', 'foo3', 'foo4', 'foo5'], name='stat'))

<xarray.DataArray (stat: 5, index: 2)>
array([[ 14.9631  ,   0.354273],
       [ 19.982272,   0.555708],
       [ 14.974026,   0.60658 ],
       [ 24.841064,   0.750451],
       [ 24.841064,   0.750451]])
Coordinates:
  * index    (index) object 'Intercept' 'Lvl'
  * stat     (stat) object 'foo1' 'foo2' 'foo3' 'foo4' 'foo5'

I want the latter, not the former, but I expected the latter when using a DataArray.

[shoyer]

Agreed, this should definitely work! (I think the fact that it doesn't is probably related to the relatively recent change that made coordinate labels along dimensions optional.)

[shoyer]

This is the location of the helper function that parsing the dim argument:

xarray/xarray/core/combine.py

Line 123 in 9763a66

def _calc_concat_dim_coord(dim):

[dcherian]

I have a simple fix in #1812 .

With that change, this works.

print(xarray.concat(data, xarray.DataArray(['foo1', 'foo2', 'foo3', 'foo4', 'foo5'], name='stat'))

But if you provided a dimension name like

print(xarray.concat(data, xarray.DataArray(['foo1', 'foo2', 'foo3', 'foo4', 'foo5'], dims=['new_dim'], name='stat'))

then new_dim is renamed to stat. What is the desired behaviour in this case viz. do we preserve dimension name new_dim or assign new name stat?

[shoyer]

What is the desired behaviour in this case viz. do we preserve dimension name new_dim or assign new name stat?

Oh, this is trickier than I thought!

The challenge is that once you make the DataArray, there is no good way to know if a default dimension name like 'dim_0' was intentional or not.

The way to handle this currently is to pass a 1-dimensional xarray.Variable object for the dim argument. These don't have separate names, so there's no ambiguity:

In [5]: xarray.concat([xarray.DataArray(1), xarray.DataArray(2)], dim=xarray.Variable('x', [3, 4]))
Out[5]:
<xarray.DataArray (x: 2)>
array([1, 2])
Coordinates:
  * x        (x) int64 3 4

But this is a little verbose. Potentially we could call xarray.as_variable() on tuple inputs (like in the Dataset constructor) so dim=('x', [3, 4]) works.

[stale]

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