issue_comments: 670594286

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https://github.com/pydata/xarray/pull/4219#issuecomment-670594286 https://api.github.com/repos/pydata/xarray/issues/4219 670594286 MDEyOklzc3VlQ29tbWVudDY3MDU5NDI4Ng== 5635139 2020-08-07T16:13:28Z 2020-08-07T16:13:28Z MEMBER

I'm still a bit confused. How does the "roll over each dimension in turn" approach equal the "roll over both dimension together" approach with a function like std? Here's a proposed counter example:

```python import xarray as xr import numpy as np

da = xr.DataArray(np.asarray([[0,10,0],[0,10,0], [0,10,0]]), dims=list('xy')) print(da)

<xarray.DataArray (x: 3, y: 3)> array([[ 0, 10, 0], [ 0, 10, 0], [ 0, 10, 0]]) Dimensions without coordinates: x, y

x_std = da.rolling(dict(x=2)).std() print(x_std)

<xarray.DataArray (x: 3, y: 3)> array([[nan, nan, nan], [ 0., 0., 0.], [ 0., 0., 0.]]) Dimensions without coordinates: x, y

x_then_y_std = x_std.rolling(dict(y=2)).std() print(x_then_y_std)

<xarray.DataArray (x: 3, y: 3)> array([[nan, nan, nan], [nan, 0., 0.], [nan, 0., 0.]]) Dimensions without coordinates: x, y

combined_std = da.rolling(dict(x=2, y=2)).std() print(combined_std)

<xarray.DataArray (x: 3, y: 3)> array([[nan, nan, nan], [nan, 5., 5.], [nan, 5., 5.]]) Dimensions without coordinates: x, y

```

 
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