issues: 955043280

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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
955043280 MDU6SXNzdWU5NTUwNDMyODA= 5644 `polyfit` with weights alters the DataArray in place 8291800 closed 0     6 2021-07-28T16:43:17Z 2023-06-09T15:38:01Z 2023-06-09T15:38:01Z CONTRIBUTOR      

What happened:

After running da.polyfit on a DataArray with weights, the data has been overwritten

What you expected to happen:

I didn't see this documented anywhere, but I did not expect that creating a polyfit dataset would clobber the original data that I'm fitting to. The data isn't altered in the case of unweighted fitting, only weighted.

Minimal Complete Verifiable Example:

```python In [2]: import xarray as xr; import numpy as np In [3]: nz, ny, nx = (10, 20, 30) In [4]: da = xr.DataArray(np.random.rand(nz, ny ,nz), dims=['z','y','x']) In [6]: da.mean(), da.max() Out[6]: (<xarray.DataArray ()> array(0.4963857), <xarray.DataArray ()> array(0.99996494))

In [7]: pf = da.polyfit("z", deg=2) # This will not alter the data In [9]: da.mean(), da.max() Out[9]: (<xarray.DataArray ()> array(0.4963857), <xarray.DataArray ()> array(0.99996494))

Non-zero w argument alters the data

In [11]: pf = da.polyfit("z", deg=2, w=np.arange(nz))

In [12]: da.mean(), da.max() Out[12]: (<xarray.DataArray ()> array(2.24317611), <xarray.DataArray ()> array(8.95963569))

```

Anything else we need to know?:

I assume it's happening here https://github.com/pydata/xarray/blob/da99a5664df4f5013c2f6b0e758394bec5e0bc80/xarray/core/dataset.py#L6805

My question is whether this is supposed to be the case to avoid copies? Or if it's accidental?

Environment:

Output of <tt>xr.show_versions()</tt> xr.show_versions() INSTALLED VERSIONS ------------------ commit: None python: 3.8.6 | packaged by conda-forge | (default, Oct 7 2020, 18:42:56) [Clang 10.0.1 ] python-bits: 64 OS: Darwin OS-release: 18.7.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: 1.10.6 libnetcdf: 4.7.4 xarray: 0.19.0 pandas: 1.1.2 numpy: 1.20.2 scipy: 1.5.2 netCDF4: 1.5.4 pydap: None h5netcdf: None h5py: 3.2.1 Nio: None zarr: 2.6.1 cftime: 1.2.1 nc_time_axis: None PseudoNetCDF: None rasterio: 1.2.1 cfgrib: None iris: None bottleneck: 1.3.2 dask: 2.14.0 distributed: 2.20.0 matplotlib: 3.3.0 cartopy: 0.18.0 seaborn: 0.10.1 numbagg: None pint: 0.16.1 setuptools: 49.6.0.post20200814 pip: 21.1.2 conda: 4.8.4 pytest: 6.2.4 IPython: 7.18.1 sphinx: 3.5.1 
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