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/4501#issuecomment-708686170,https://api.github.com/repos/pydata/xarray/issues/4501,708686170,MDEyOklzc3VlQ29tbWVudDcwODY4NjE3MA==,1053153,2020-10-14T22:07:38Z,2020-10-14T22:07:38Z,CONTRIBUTOR,"My mental model of what's happening may not be correct.  I did want `sel()`, `isel()`, and `squeeze()` to all operate the same way (and maybe someday even work on non-dim coordinates!).  Replacing `squeeze()` with `isel()` in my initial example gives the same failure, which I would want it to work:

```
import numpy as np
import xarray as xr

arr1 = xr.DataArray(np.zeros((1,5)), dims=['y', 'x'], coords={'e': ('y', [10])})
arr2 = arr1.isel(y=0).expand_dims('y')
xr.testing.assert_identical(arr1, arr2)
```

```
AssertionError: Left and right DataArray objects are not identical

Differing coordinates:
L   e        (y) int64 10
R   e        int64 10
```

The non-dim coordinate `e` has forgotten that it was associated with `y`.  I'd prefer that this association remained.

Where it gets really interesting is in the following example where the non-dim coordinate moves from one dim to another.  I understand the logic here (since the `isel()` were done in a way that correlates 'y' and 'z').  In my proposal, this would not happen without explicit user intervention -- which may actually be desired here (it's sorta surprising):

```
import numpy as np
import xarray as xr

arr = xr.DataArray(np.zeros((2, 2, 5)), dims=['z', 'y', 'x'], coords={'e': ('y', [10, 20])})
print(arr.coords)
print()

arr0 = arr.isel(z=0,y=0)
arr1 = arr.isel(z=1,y=1)

arr_concat = xr.concat([arr0, arr1], 'z')
print(arr_concat.coords)
```

```
Coordinates:
    e        (y) int64 10 20

Coordinates:
    e        (z) int64 10 20
```

","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,718716799
https://github.com/pydata/xarray/issues/4501#issuecomment-708561579,https://api.github.com/repos/pydata/xarray/issues/4501,708561579,MDEyOklzc3VlQ29tbWVudDcwODU2MTU3OQ==,1053153,2020-10-14T17:51:15Z,2020-10-14T17:51:15Z,CONTRIBUTOR,"> One problem with this -- at least for now -- is that xarray currently doesn't allow coordinates on DataArray objects to have dimensions that don't appear on the DataArray itself.

Ah, then that would be the desire here.

> It might also be surprising that this would make `squeeze('y')` inconsistent with `isel(y=0)`

The suggestion here is that both of these would behave the same.  The MCVE was just for the squeeze case, but I expect that `isel` and `sel` would both allow non-dim coords to maintain the reference to their original dim (even if it becomes a non-dim coord itself).
","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,718716799
https://github.com/pydata/xarray/issues/4501#issuecomment-706640485,https://api.github.com/repos/pydata/xarray/issues/4501,706640485,MDEyOklzc3VlQ29tbWVudDcwNjY0MDQ4NQ==,1053153,2020-10-11T02:37:00Z,2020-10-11T02:37:00Z,CONTRIBUTOR,"I'm not a huge fan of adding arguments for a case that rarely comes up (I presume).  

One difference in your example is that the 'e' coord is never based on 'y', so I would not want it expanded -- so I'd still like that test to pass.

The case I'm interested in is where the non-dimension coords are based on existing dimension coords that gets squeezed.

So in this example:
```
import numpy as np
import xarray as xr

arr1 = xr.DataArray(np.zeros((1,5)), dims=['y', 'x'], coords={'y': [42], 'e': ('y', [10])})
arr1.squeeze()
```

The squeezed array looks like:

```
<xarray.DataArray (x: 5)>
array([0., 0., 0., 0., 0.])
Coordinates:
    y        int64 42
    e        int64 10
Dimensions without coordinates: x
```

What I think would be more useful:

```
<xarray.DataArray (x: 5)>
array([0., 0., 0., 0., 0.])
Coordinates:
    y        int64 42
    e        (y)  int64 10     <---- Note the (y)
Dimensions without coordinates: x
```
","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,718716799