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-708588777,https://api.github.com/repos/pydata/xarray/issues/4501,708588777,MDEyOklzc3VlQ29tbWVudDcwODU4ODc3Nw==,1217238,2020-10-14T18:41:21Z,2020-10-14T18:41:32Z,MEMBER,"If we changed `isel()` to only modify data variables, then we would be in trouble with something like `isel(x=slice(3))`. The coordinate system would be inconsistent if we slice only the data variables but not the coordinates.
There's a bit of a conflict here between two desirable properties:
1. Methods on a Dataset only modify data variables, leaving coordinates unchanged
2. Methods on a Dataset keep the entire coordinate system for the Dataset consistent, including coordinates","{""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-708167473,https://api.github.com/repos/pydata/xarray/issues/4501,708167473,MDEyOklzc3VlQ29tbWVudDcwODE2NzQ3Mw==,1217238,2020-10-14T05:33:59Z,2020-10-14T05:33:59Z,MEMBER,"> 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
> ```
Thanks for clarifying!
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.
It might also be surprising that this would make `squeeze('y')` inconsistent with `isel(y=0)`","{""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
https://github.com/pydata/xarray/issues/4501#issuecomment-706639713,https://api.github.com/repos/pydata/xarray/issues/4501,706639713,MDEyOklzc3VlQ29tbWVudDcwNjYzOTcxMw==,1217238,2020-10-11T02:26:42Z,2020-10-11T02:26:42Z,MEMBER,"Hi @chrisroat, thanks for the clear bug report!
It indeed be nice if `squeeze` followed by `expand_dims` preserved the original inputs, but I don't think that is possible in general -- the `squeeze` operation removes information.
For example, this array *does* currently satisfy your desired property, but wouldn't if we made the change you request:
```python
arr1 = xr.DataArray(np.zeros((1,5)), dims=['y', 'x'], coords={'e': 10})
arr2 = arr1.squeeze('y').expand_dims('y')
xr.testing.assert_identical(arr1, arr2) # passes
```
I suspect our best option for achieving this behavior would be to add another optional argument to `expand_dims`, e.g., perhaps
- `expand_dims(..., expand_coords=False)`: don't expand coordinates (default behavior)
- `expand_dims(..., expand_coords=True)`: expand all coordinates
- `expand_dims(..., expand_coords=['e'])`: only expand the coordinate `'e'`
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