具有两次多索引的stack xarray.DataArray返回unformati

Question

我尝试在同一数组上使用xr.DataArray.stack()方法两次，将一个多索引堆叠到新的多索引：

import numpy as np
import xarray as xr

da = xr.DataArray(np.random.rand(4,4,4,4), 
     [('latitude', range(4)), ('longitude', range(4)), 
      ('variables', range(4)), ('time', range(4))])

da.stack(dim1=('latitude','longitude')).stack(dim2=('variables','time')) # this works fine
da.stack(dim1=('latitude','longitude')).stack(dim2=('dim1','time'))

最后一行产生以下错误：

  File "/usr/local/Miniconda3-envs/envs/2018/envs/iacpy3_2018/lib/python3.6/site-packages/xarray/core/dataarray.py", line 1102, in stack
    ds = self._to_temp_dataset().stack(**dimensions)
  File "/usr/local/Miniconda3-envs/envs/2018/envs/iacpy3_2018/lib/python3.6/site-packages/xarray/core/dataset.py", line 2101, in stack
    result = result._stack_once(dims, new_dim)
  File "/usr/local/Miniconda3-envs/envs/2018/envs/iacpy3_2018/lib/python3.6/site-packages/xarray/core/dataset.py", line 2070, in _stack_once
    idx = utils.multiindex_from_product_levels(levels, names=dims)
  File "/usr/local/Miniconda3-envs/envs/2018/envs/iacpy3_2018/lib/python3.6/site-packages/xarray/core/utils.py", line 80, in multiindex_from_product_levels
    return pd.MultiIndex(levels, labels, sortorder=0, names=names)
  File "/usr/local/Miniconda3-envs/envs/2018/envs/iacpy3_2018/lib/python3.6/site-packages/pandas/core/indexes/multi.py", line 132, in __new__
    result._set_names(names)
  File "/usr/local/Miniconda3-envs/envs/2018/envs/iacpy3_2018/lib/python3.6/site-packages/pandas/core/indexes/multi.py", line 570, in _set_names
    self.levels[l].rename(name, inplace=True)
  File "/usr/local/Miniconda3-envs/envs/2018/envs/iacpy3_2018/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 1167, in set_names
    raise TypeError("Must pass list-like as `names`.")
TypeError: Must pass list-like as `names`.

动机：

我的原始数组与上面的玩具数据集具有相同的坐标，但维度不同。但是，要使用sklearn.ensemble.RandomForestRegressor，我需要在数据点的形状（平坦的纬度、经度和时间）和变量中使用数据，且不缺少值。因此，我想先堆叠纬度和经度，移除所有海洋点，因为它们填充了np.nan，然后使用新创建的多重索引堆叠时间

问题：

在我看来，此错误似乎未实现该功能

1. 如果是，原因为何
2. 如果是，为什么没有更详细的错误消息
3. 在xarray内是否有解决问题的方法（即，不将数据转换为numpy数组或类似的方法）