issue_comments: 258560862

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https://github.com/pydata/xarray/issues/1077#issuecomment-258560862 https://api.github.com/repos/pydata/xarray/issues/1077 258560862 MDEyOklzc3VlQ29tbWVudDI1ODU2MDg2Mg== 17055041 2016-11-04T22:14:05Z 2016-11-04T22:14:05Z NONE

So if I'm properly understanding and synthesizing your ( @benbovy and @shoyer ) comments: We want the hybrid format for maximum compatibility, with the MultiIndex split into separate 1D raw value coordinates. Using the example above, these would be [1, 1, 2, 2, 3, 3] and ['a', 'b', 'a', 'b', 'a', 'b']. The information about which coordinates are in a MultiIndex (and their order) gets saved in an attribute on the data in the file, like data.attrs['multiindex_levels'] = 'numbers letters'. So 3rd-party tools (or older xarray) will have the non-MultiIndex coords to use, but newer xarray will see the 'multiindex_levels' and automatically reconstruct the MultiIndex when the file is read.

@shoyer , I see what you mean about Variable or future DataArrays not needing a placeholder index. Would that still be backwards-compatible with older xarrays if a saved DataArray has one dim that is a MultiIndex and other dims that are not?

@benbovy , what does the encoding attribute do? It seems to me that, for a DataArray that's already created or loaded, xarray knows about its MultiIndexes and could do the right thing while writing to the backend file without being told to. Are you referring to the metadata in the file (like 'multiindex_levels') that ensures proper interpretation and automatic reconstruction when reading?

 
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