issue_comments: 630912785

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https://github.com/pydata/xarray/issues/4077#issuecomment-630912785 https://api.github.com/repos/pydata/xarray/issues/4077 630912785 MDEyOklzc3VlQ29tbWVudDYzMDkxMjc4NQ== 1217238 2020-05-19T15:54:02Z 2020-05-19T15:54:02Z MEMBER

That was actually deliberate, xr.combine_by_coords is only checking the first value of each coord is different, to avoid loading big coordinates into memory. (see this line) As the first y value is 0 in both cases it's just saying "we have a match!" and overwriting.

We already have the coordinates loaded into memory at this point -- each elements of indexes is a pandas.Index.

Looking at the first values makes sense for determining the order, but doesn't guarantee that they are safe to concatenate. The contract of I think we are missing another safety check verifying indexes[i][-1] <= indexes[i+1][0] for all indexes in order, in a way that handles ties correctly.

In my opinion, xarray's combine functions like combine_by_coords should never override values, unless an unsafe option was explicitly chosen.

 
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