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
932393011,MDExOlB1bGxSZXF1ZXN0Njc5NzQyODk1,5548,Fix 2 type annotations in dataarray.py,28426758,closed,0,,,2,2021-06-29T09:03:23Z,2021-06-29T19:15:33Z,2021-06-29T18:51:19Z,CONTRIBUTOR,,0,pydata/xarray/pulls/5548,"Both `set_index` and `reset_index` are wrappers to other methods that return `""DataArray""`, not `Optional[""DataArray""]`. That is, they will never return None. That's why these methods should also have only `""DataArray""` in there return signature. This way it will be possible to do something like `myarray = myarray.reset_index(...)` without getting a complaint from Mypy.

For extended discussion, see https://github.com/pydata/xarray/issues/5533#issuecomment-869822366

<!-- Feel free to remove check-list items aren't relevant to your change -->

- [x] Closes #5533
- [ ] Tests added
- [ ] Passes `pre-commit run --all-files`
- [ ] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
- [ ] New functions/methods are listed in `api.rst`
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