html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/pull/3598#issuecomment-562332357,https://api.github.com/repos/pydata/xarray/issues/3598,562332357,MDEyOklzc3VlQ29tbWVudDU2MjMzMjM1Nw==,2448579,2019-12-05T21:44:31Z,2019-12-05T21:44:31Z,MEMBER,"> But I believe we assume that when all the values in dependencies are themselves keys of layers. 

I came to the same conclusion when I looked at it but didn't know if this was by accident or design. The docs don't say anything.

anyway the fix looks good. Thanks for tackling this.

I'll merge tomorrow if there are no other comments.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,533555794
https://github.com/pydata/xarray/pull/3598#issuecomment-562310739,https://api.github.com/repos/pydata/xarray/issues/3598,562310739,MDEyOklzc3VlQ29tbWVudDU2MjMxMDczOQ==,1312546,2019-12-05T20:47:02Z,2019-12-05T20:47:02Z,MEMBER,"Hopefully the new comments make sense. I'm struggling a bit to explain things since I don't fully understand them myself :)

> So it was a graph construction issue.

I *think* so. Dask doesn't actually validate arguments passed to HighLevelGraph. But I believe we assume that when all the values in `dependencies` are themselves keys of `layers`. We didn't have that before with things like

```
(Pdb) pp collections[0].dask.dependencies
{'all-84bc51ac43a9275b3662b0089710eab9': {'or_-64f95b81b2f8001b4c61f2023ac4c223'},
 ...
 'eq-abac622d95ce5055d3e7b7dea944ec37': {'lambda-e79de3edfa267f41111057d26471bce3-x',
                                         'ones-c4a83f4b990021618d55e0fa61a351d6'},
...
}
 ```

The `'lambda-e79de3edfa267f41111057d26471bce3-x'` wasn't a layer of the graph. It was previously nested under the single new layer we were creating `gname` or `lambda-e79de3edfa267f41111057d26471bce3` in this case.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,533555794