issues: 340316108

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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
340316108 MDU6SXNzdWUzNDAzMTYxMDg= 2280 string coords are converted to object dtype when using MultiIndex / stacking 1034035 closed 0     5 2018-07-11T16:26:12Z 2022-09-27T12:12:06Z 2022-09-27T12:12:05Z NONE      

Code Sample

```python import xarray as xr import pandas as pd

indx = pd.MultiIndex.from_arrays([['1', '2', '1', '2'], ['100', '100', '200', '200']], names=['a', 'b'])

MultiIndex(levels=[['1', '2'], ['100', '200']],

labels=[[0, 1, 0, 1], [0, 0, 1, 1]],

names=['a', 'b'])

tst = xr.DataArray(data=np.random.random((4)), coords=[('ab', indx)])

<xarray.DataArray (ab: 4)>

array([0.754308, 0.268363, 0.799407, 0.513185])

Coordinates:

* ab (ab) MultiIndex

- a (ab) object '1' '2' '1' '2'

- b (ab) object '100' '100' '200' '200'

tst2 = xr.DataArray(data=np.random.random((2,2)), coords=[('a', ['1', '2']), ('b', ['100', '200'])])

<xarray.DataArray (a: 2, b: 2)>

array([[0.854303, 0.179755],

[0.463453, 0.478913]])

Coordinates:

* a (a) <U1 '1' '2'

* b (b) <U3 '100' '200'

tst2.stack(ab=('a', 'b'))

<xarray.DataArray (ab: 4)>

array([0.854303, 0.179755, 0.463453, 0.478913])

Coordinates:

* ab (ab) MultiIndex

- a (ab) object '1' '1' '2' '2'

- b (ab) object '100' '200' '100' '200'

```

Problem description

The dtype for a and b in tst and tst2.stack() is object rather than some string dtype.

Expected Output

A string dtype for a and b.

Output of xr.show_versions()

# Paste the output here xr.show_versions() here INSTALLED VERSIONS ------------------ commit: None python: 3.6.5.final.0 python-bits: 64 OS: Linux OS-release: 4.15.0-24-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 xarray: 0.10.7 pandas: 0.23.1 numpy: 1.14.5 scipy: 1.1.0 netCDF4: 1.4.0 h5netcdf: 0.6.1 h5py: 2.8.0 Nio: None zarr: None bottleneck: 1.2.1 cyordereddict: None dask: 0.18.1 distributed: 1.22.0 matplotlib: 2.2.2 cartopy: None seaborn: 0.8.1 setuptools: 39.2.0 pip: 9.0.3 conda: 4.5.5 pytest: 3.6.2 IPython: 6.4.0 sphinx: 1.7.5 
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   completed 13221727 issue

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