issue_comments: 789737571

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https://github.com/pydata/xarray/pull/4979#issuecomment-789737571 https://api.github.com/repos/pydata/xarray/issues/4979 789737571 MDEyOklzc3VlQ29tbWVudDc4OTczNzU3MQ== 4160723 2021-03-03T14:05:19Z 2021-03-03T14:17:19Z MEMBER

Alignment hasn't been discussed yet here, but it should! Some quick thoughts:

  • support for alignment should probably be optional for an Xarray index wrapper.
  • like pandas.Index, the index wrapper classes that support it should implement .equals(), .union() and/or .intersection()
  • support might be partial if that makes sense (outer, inner, left, right, exact...).
  • index equality might involve more than just the labels, like the CRSIndex proposed in #2996
  • some indexes might implement inexact alignment, like in #4489 or a KDTree index that selects nearest-neighbors within a given tolerance
  • alignment may be "multi-dimensional", i.e., the KDTree example above vs. dimensions aligned independently of each other
  • we need to decide what to do when one dimension has more than one index that supports alignment
  • we should probably raise unless the user explicitly specify which index to use for the alignment
  • we need to decide what to do when one dimension has one or more index(es) but none support alignment
  • either we raise or we fail back (silently) to alignment based on dimension size
  • for inexact alignment, the tolerance threshold might be given when building the index and/or when performing the alignment
  • are there cases where we want a specific index to perform alignment and another index to perform selection? It would be tricky to support that unless we allow multiple indexes per coordinate. "Meta" indexes (https://github.com/pydata/xarray/pull/4979#discussion_r585203065) would help but then I'm worried about the possible explosion of index wrapper classes. 
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