issue_comments: 280441902

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https://github.com/pydata/xarray/pull/924#issuecomment-280441902 https://api.github.com/repos/pydata/xarray/issues/924 280441902 MDEyOklzc3VlQ29tbWVudDI4MDQ0MTkwMg== 1217238 2017-02-16T20:00:44Z 2017-02-16T20:00:44Z MEMBER

@RafalSkolasinski Sure, here is the current list:

  1. The implementation should probably switch to use set_index() (after stacking) to combine groupby arguments into a single MultiIndex, instead of the current version which constructs the MultiIndex explicitly in GroupBy.__init__, duplicating a lot of the set_index code.
  2. We need to consider how to handle multiple group-by arguments along orthogonal dimensions. For example, consider the 4x4 array from the gist at the top of this PR where grouping is done over 2x2 blocks. With the current PR, the array is flattened, so iteration is done over flat arrays of length 4, but we probably want to actually iterate over 2x2 arrays that match the shape of the original data. This could be done either by stacking/unstacking at each iteration step (slow, but would work) or by making groupby truly do indexing and concatenation over multiple dimensions at once (certainly faster and more elegant, but significantly more work to implement).
  3. The change also needs to be rebased to account for changes on master, but by the time you account for the above changes there may not be much of the current version remaining.
  4. We also need comprehensive test coverage for the desired behavior.
 
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