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
438947247,MDU6SXNzdWU0Mzg5NDcyNDc=,2933,Stack() & unstack() issues on Multindex,1559890,closed,0,,,4,2019-04-30T19:47:51Z,2022-04-09T15:23:28Z,2022-04-09T15:23:28Z,NONE,,,,"I would like to reshape the DataArray by one level in the Multindex, and I thought the `stack()`/`unstack()` should be the solution.

Make a DataArray with Multindex:
```python
import pandas as pd
arrays = [np.array(['bar', 'bar', 'baz', 'baz', 'foo', 'foo']),
          np.array(['one', 'two', 'one', 'two', 'one', 'two'])]
da = pd.DataFrame(np.random.randn(6, 4)).to_xarray().to_array()
da.coords['index'] = pd.MultiIndex.from_arrays(arrays, names=['first', 'second'])
da
```
```
<xarray.DataArray (variable: 4, index: 6)>
array([[ 0.379189,  1.082292, -2.073478, -0.84626 , -1.529927, -0.837407],
       [-0.267983, -0.2516  , -1.016653, -0.085762, -0.058382, -0.667891],
       [-0.013488, -0.855332, -0.038072, -0.385211, -2.149742, -0.304361],
       [ 1.749561, -0.606031,  1.914146,  1.6292  , -0.515519,  1.996283]])
Coordinates:
  * index     (index) MultiIndex
  - first     (index) object 'bar' 'bar' 'baz' 'baz' 'foo' 'foo'
  - second    (index) object 'one' 'two' 'one' 'two' 'one' 'two'
  * variable  (variable) int32 0 1 2 3
```
### Stack problem:
I want a dimension merges into another one:
```python
da.stack({'index':['variable']})
```
```
ValueError: cannot create a new dimension with the same name as an existing dimension
```

### Unstack problem:
Unstacking by the whole Multindex worked:
```python
da.unstack('index')
```
```
<xarray.DataArray (variable: 4, first: 3, second: 2)>
array([[[ 0.379189,  1.082292],
        [-2.073478, -0.84626 ],
        [-1.529927, -0.837407]],

       [[-0.267983, -0.2516  ],
        [-1.016653, -0.085762],
        [-0.058382, -0.667891]],

       [[-0.013488, -0.855332],
        [-0.038072, -0.385211],
        [-2.149742, -0.304361]],

       [[ 1.749561, -0.606031],
        [ 1.914146,  1.6292  ],
        [-0.515519,  1.996283]]])
Coordinates:
  * variable  (variable) int32 0 1 2 3
  * first     (first) object 'bar' 'baz' 'foo'
  * second    (second) object 'one' 'two'
```
But unstacking by a specified level failed:
```python
da.unstack('first')
```
```
ValueError: Dataset does not contain the dimensions: ['first']
```","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2933/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue
602793814,MDU6SXNzdWU2MDI3OTM4MTQ=,3984,Support flexible DataArray shapes in Dataset ,1559890,closed,0,,,4,2020-04-19T18:57:07Z,2020-09-26T00:02:12Z,2020-09-26T00:02:12Z,NONE,,,,"I always use Pandas to deal with my neuroscience data (multi-dimension). It is annoying to stack and unstack all the time and I heard Xarray is designed for multi-dimension data. 

In neuroscience research, we usually have multiple participants and we will test them different times, which means the data may look like this:

- participant A:   
  - 2\*5\*100 matrix
- participant B:   
  - 2\*5\*101 matrix

(100 and 101 are the testing times)

But **Dataset doesn't support to have 2\*5\*100 DataArray and 2\*5\*101 DataArray together**. Is there any solution to deal with that kind of data in Xarray?","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3984/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue