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/issues/3158#issuecomment-514118022,https://api.github.com/repos/pydata/xarray/issues/3158,514118022,MDEyOklzc3VlQ29tbWVudDUxNDExODAyMg==,35968931,2019-07-23T08:53:50Z,2019-07-23T08:53:50Z,MEMBER,"Thanks @zdgriffith. Yes it would be clearer to make that distinction. How about just adding a line:

```
concat_dim : str, or list of str, DataArray, Index or None, optional
        Dimensions to concatenate files along. You only
        need to provide this argument if any of the dimensions along which you
        want to concatenate is not a dimension in the original datasets, e.g.,
        if you want to stack a collection of 2D arrays along a third dimension.
        Required for combine='nested', but not used for combine='by_coords'.
        ...
```

> as None is a valid input, wouldn't it make more sense as an optional argument?

The reason it's not an optional argument to `combine_nested` is because currently `None` tells `combine_nested` to do something specific: merge datasets along that axis of the input instead of concatenating them. This is useful when you want to combine by some combination of concatenation and merging.

That's what 

```
Set concat_dim=[..., None, ...] explicitly to disable concatenation along a particular dimension.
```

is supposed to be getting at, but obviously also needs to be clearer. Note `None` is only a valid input because it gets converted to `[None]` internally for convenience.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,471409673