issue_comments: 509518811

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https://github.com/pydata/xarray/issues/3037#issuecomment-509518811 https://api.github.com/repos/pydata/xarray/issues/3037 509518811 MDEyOklzc3VlQ29tbWVudDUwOTUxODgxMQ== 32473508 2019-07-09T07:09:05Z 2019-07-09T07:14:44Z CONTRIBUTOR

@shoyer I would like to work on this issue (hopefully my first contribution).

I believe the error should indicate that we cannot groupby an empty group. This would be consistent with the documentation:

``` DataArray.groupby(group, squeeze: bool = True, restore_coord_dims: Optional[bool] = None) Parameters groupstr, DataArray or IndexVariable Array whose unique values should be used to group this array. If a string, must be the name of a variable contained in this dataset. ````

For the groupby class, we can raise an error in case the groupby object instantiation does not specify values to be grouped by. In the case above:

``` ValueError: variable to groupby must not be empty

```

If you find such a solution acceptable, I would create a pull request.

Thank you in advance

 
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