issues: 826990294

What happened:

If a DataArray is reindex() with a list, the Series created from DataArray.to_series() has an index with no name in the reindexed dimension. It was working for 2+ dimension array in 0.16.2 (i.e. the Series has MultiIndex instead of Index), but 0.17.0 doesn't work. For 1 dimension array, both 0.16.2 and 0.17.0 are not working as expected.

What you expected to happen:

I expect the Index.names of Series created from DataArray.to_series() contains the name of the dimensions of the DataArray, no matter reindex() was called on the DataArray before or not.

Minimal Complete Verifiable Example:

```python

import numpy as np import pytest import xarray as xr

def test_multiindex(): array = xr.DataArray( np.arange(12).reshape((4, 3)), coords=[ ("foo", ["a", "b", "c", "d"]), ("bar", ["x", "y", "z"]), ] ) assert array.to_series().index.names == ["foo", "bar"]

def test_multiindex_reindex(): array = xr.DataArray( np.arange(12).reshape((4, 3)), coords=[ ("foo", ["a", "b", "c", "d"]), ("bar", ["x", "y", "z"]), ] ) series = array.reindex(bar=["x", "y"]).to_series() assert series.index.names == ["foo", "bar"]

def test_index_str(): array = xr.DataArray(np.arange(4), coords=[("foo", ["a", "b", "c", "d"])]) assert array.to_series().index.names == ["foo"]

def test_index_str_reindex(): array = xr.DataArray(np.arange(4), coords=[("foo", ["a", "b", "c", "d"])]) series = array.reindex(foo=["a", "b", "c"]).to_series() assert series.index.names == ["foo"]

def test_index_int(): array = xr.DataArray(np.arange(4), coords=[("foo", [1, 2, 3, 4])]) assert array.to_series().index.names == ["foo"]

def test_index_int_reindex(): array = xr.DataArray(np.arange(4), coords=[("foo", [1, 2, 3, 4])]) series = array.reindex(foo=[1, 2, 3]).to_series() assert series.index.names == ["foo"]

```

Version 0.16.2:

It works as expected for MultiIndex. However, the index name is missing after reindex for 1 dimension array.

``` ================================================= test session starts ================================================= platform win32 -- Python 3.8.6, pytest-6.2.2, py-1.10.0, pluggy-0.13.1 plugins: cov-2.11.1 collected 6 items

tests\test_to_series_index.py ...F.F [100%]

====================================================== FAILURES ======================================================= _______ test_index_str_reindex __________

def test_index_str_reindex():
    array = xr.DataArray(np.arange(4), coords=[("foo", ["a", "b", "c", "d"])])
    series = array.reindex(foo=["a", "b", "c"]).to_series()

  assert series.index.names == ["foo"]

E AssertionError: assert FrozenList([None]) == ['foo'] E At index 0 diff: None != 'foo' E Use -v to get the full diff

tests\test_to_series_index.py:39: AssertionError _______ test_index_int_reindex __________

def test_index_int_reindex():
    array = xr.DataArray(np.arange(4), coords=[("foo", [1, 2, 3, 4])])
    series = array.reindex(foo=[1, 2, 3]).to_series()

  assert series.index.names == ["foo"]

E AssertionError: assert FrozenList([None]) == ['foo'] E At index 0 diff: None != 'foo' E Use -v to get the full diff

tests\test_to_series_index.py:50: AssertionError =============================================== short test summary info =============================================== FAILED tests/test_to_series_index.py::test_index_str_reindex - AssertionError: assert FrozenList([None]) == ['foo'] FAILED tests/test_to_series_index.py::test_index_int_reindex - AssertionError: assert FrozenList([None]) == ['foo'] ============================================= 2 failed, 4 passed in 0.61s ============================================= ```

Version 0.17.0:

Index name is missing after reindex on both single level Index and MultiIndex.

``` ================================================= test session starts ================================================= platform win32 -- Python 3.8.6, pytest-6.2.2, py-1.10.0, pluggy-0.13.1 plugins: cov-2.11.1 collected 6 items

tests\test_to_series_index.py .F.F.F [100%]

====================================================== FAILURES ======================================================= _______ test_multiindex_reindex _________

def test_multiindex_reindex():
    array = xr.DataArray(
        np.arange(12).reshape((4, 3)),
        coords=[
            ("foo", ["a", "b", "c", "d"]),
            ("bar", ["x", "y", "z"]),
        ]
    )
    series = array.reindex(bar=["x", "y"]).to_series()

  assert series.index.names == ["foo", "bar"]

E AssertionError: assert FrozenList(['foo', None]) == ['foo', 'bar'] E At index 1 diff: None != 'bar' E Use -v to get the full diff

tests\test_to_series_index.py:28: AssertionError _______ test_index_str_reindex __________

def test_index_str_reindex():
    array = xr.DataArray(np.arange(4), coords=[("foo", ["a", "b", "c", "d"])])
    series = array.reindex(foo=["a", "b", "c"]).to_series()

  assert series.index.names == ["foo"]

E AssertionError: assert FrozenList([None]) == ['foo'] E At index 0 diff: None != 'foo' E Use -v to get the full diff

tests\test_to_series_index.py:39: AssertionError _______ test_index_int_reindex __________

def test_index_int_reindex():
    array = xr.DataArray(np.arange(4), coords=[("foo", [1, 2, 3, 4])])
    series = array.reindex(foo=[1, 2, 3]).to_series()

  assert series.index.names == ["foo"]

E AssertionError: assert FrozenList([None]) == ['foo'] E At index 0 diff: None != 'foo' E Use -v to get the full diff

tests\test_to_series_index.py:50: AssertionError =============================================== short test summary info =============================================== FAILED tests/test_to_series_index.py::test_multiindex_reindex - AssertionError: assert FrozenList(['foo', None]) == [... FAILED tests/test_to_series_index.py::test_index_str_reindex - AssertionError: assert FrozenList([None]) == ['foo'] FAILED tests/test_to_series_index.py::test_index_int_reindex - AssertionError: assert FrozenList([None]) == ['foo'] ============================================= 3 failed, 3 passed in 0.61s ============================================= ```

Environment:

Output of <tt>xr.show_versions()</tt> for 0.16.2

commit: None python: 3.8.6 (tags/v3.8.6:db45529, Sep 23 2020, 15:52:53) [MSC v.1927 64 bit (AMD64)] python-bits: 64 OS: Windows OS-release: 10 machine: AMD64 processor: Intel64 Family 6 Model 158 Stepping 13, GenuineIntel byteorder: little LC_ALL: None LANG: None LOCALE: English_United States.1252 libhdf5: None libnetcdf: None xarray: 0.16.2 pandas: 1.2.3 numpy: 1.20.1 scipy: None netCDF4: None pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: None nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: None distributed: None matplotlib: None cartopy: None seaborn: None numbagg: None pint: None setuptools: 53.0.0 pip: 21.0.1 conda: None pytest: 6.2.2 IPython: 7.21.0 sphinx: None

Output of <tt>xr.show_versions()</tt> for 0.17.0

commit: None python: 3.8.6 (tags/v3.8.6:db45529, Sep 23 2020, 15:52:53) [MSC v.1927 64 bit (AMD64)] python-bits: 64 OS: Windows OS-release: 10 machine: AMD64 processor: Intel64 Family 6 Model 158 Stepping 13, GenuineIntel byteorder: little LC_ALL: None LANG: None LOCALE: English_United States.1252 libhdf5: None libnetcdf: None xarray: 0.17.0 pandas: 1.2.3 numpy: 1.20.1 scipy: None netCDF4: None pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: None nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: None distributed: None matplotlib: None cartopy: None seaborn: None numbagg: None pint: None setuptools: 53.0.0 pip: 21.0.1 conda: None pytest: 6.2.2 IPython: 7.21.0 sphinx: None