BUG: Dataset constructor puts lists in coords rather that data_vars

[max-sixty]

I'd expect a to be a data_vars rather than a coord here:

In [9]: xr.Dataset(data_vars={'a': [2,3]}, attrs={'name':'hello'})
Out[9]: 
<xarray.Dataset>
Dimensions:  (a: 2)
Coordinates:
  * a        (a) int64 2 3
Data variables:
    *empty*
Attributes:
    name: hello

[shoyer]

This may not be clearly documented, but it is/was intentional behavior.

To be clear, there are two things going on here:

1. Arguments like {'a': [2, 3]} get implicit dimensions added, e.g., {'a': ('a', [2, 3])}.
2. 1D data_vars arguments which have the same name as their sole argument are converted in coords.

Originally, this made sense because we didn't have a separate coords argument. Now, this could simply be passed as coords instead, which is certainly more explicit. So instead of allowing this we could raise an error. My concern such a deprecation this is that (1) there's a lot of code written with the assumption that this works and (2) there's not much be to be gained -- there's nothing else that we could sensibly use this syntax for, and (3) this is marginally convenient, allowing for slightly less typing (one less pair of curly braces).

[max-sixty]

Closing - while the behavior could be surprising at first glance, for @shoyer 's reasons above: not much to gain from changes