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
831008649,MDExOlB1bGxSZXF1ZXN0NTkyNDM0MDEy,5033,Allow using a custom engine class directly in xr.open_dataset,14371165,closed,0,,,13,2021-03-13T22:12:39Z,2021-05-18T18:16:28Z,2021-04-15T02:02:04Z,MEMBER,,0,pydata/xarray/pulls/5033,"<!-- Feel free to remove check-list items aren't relevant to your change -->

- [x] Closes #4838
- [x] Tests added
- [x] Passes `pre-commit run --all-files`

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791725552,MDU6SXNzdWU3OTE3MjU1NTI=,4838,Simplify adding custom backends,14371165,closed,0,,,0,2021-01-22T06:02:53Z,2021-04-15T02:02:03Z,2021-04-15T02:02:03Z,MEMBER,,,,"<!-- Please do a quick search of existing issues to make sure that this has not been asked before. -->

**Is your feature request related to a problem? Please describe.**
I've been working on opening custom hdf formats in xarray, reading up on the apiv2 it is currently only possible to declare a new external plugin in setup.py but that doesn't seem easy or intuitive to me.

**Describe the solution you'd like**
Why can't we simply be allowed to add functions to the engine parameter? Example:
```python
from custom_backend import engine

ds = xr.load_dataset(filename, engine=engine)
```
This seems like a small function change to me from my initial _quick_ look because there's mainly a bunch of string checks in the normal case until we get to the registered backend functions, if we send in a function instead in the engine-parameter we can just bypass those checks.
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