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id node_id number title user state locked assignee milestone comments created_at updated_at ▲ closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
490228661 MDExOlB1bGxSZXF1ZXN0MzE0ODc0MDI5 3283 Add hypothesis test for netCDF4 roundtrip takluyver 327925 open 0     5 2019-09-06T09:33:48Z 2022-11-21T22:45:13Z   MEMBER   0 pydata/xarray/pulls/3283

Part of #1846: add a property-based test for reading & writing netCDF4 files.

This is the first time I've played with Hypothesis, but it seems to be working - e.g. I got an error with float16, and the netCDF docs show that 16-bit floats are not a supported data type.

However:

  • This currently only tests a dataset with a single variable - it could be extended to multiple variables if that's useful.
  • It looks like netCDF4 should support unicode characters, but it failed when I didn't have max_codepoint=255 in there. I don't know if that's an expected limitation I'm not aware of, or a bug somewhere. But I thought I'd make the test pass for now.
 
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     xarray 13221727 pull
568968607 MDU6SXNzdWU1Njg5Njg2MDc= 3786 DataArray.unstack() leaving dimensions 'in order' takluyver 327925 open 0     9 2020-02-21T13:53:40Z 2020-02-23T04:03:06Z   MEMBER      

It's documented that when you .unstack() a DataArray, "New dimensions will be added at the end."

I'm using .unstack() kind of like np.reshape(), to split one dimension into two. From this perspective, it's surprising that new dimensions go at the end, rather than in place of the old dimension. This also means that either the new array is no longer C-contiguous, or the .unstack() operation has had to copy all the data to rearrange it. It appears I'm not the only one to think this: the code sample in #2419 (about adding a reshape method) does the same workaround that I do: unstack and then carefully reorder the dimensions.

First, I'd like to understand: why the end? It seems like there's an obvious place to put unstacked dimensions (ab, c -> a, b, c), but presumably there's some reason it doesn't work that way.

Then, is there anything one can do with xarray today to 'unstack' a dimension while keeping the order, short of using .transpose() afterwards to recreate the desired order? And if not, is there any potential to add a feature for that?

I guess this might touch on missing data: unstack() can create and handle 'gaps', whereas reshape() can't. If so, is there some more limited operation that could only unstack a perfectly regular multiindex - where it's guaranteed to separate into 2 dimensions without creating gaps?

 
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     xarray 13221727 issue

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