COMPAT: MultiIndex checking is fragile

[jreback]

https://github.com/pydata/xarray/blob/master/xarray/core/dataset.py#L2755

checking for a MI via

if hasattr(idx, 'levels'):
    # is a MI

should be changed to

if idx.nlevels > 1:
    # is a MI

see here. This is not currently implemented but good to get ahead of this hard to detect usage of the API.

[shoyer]

Thanks for the heads up.

For our purposes, I think it would actually be just fine to use isinstance(idx, pd.MultiIndex) here.