issues: 648981227

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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
648981227 MDU6SXNzdWU2NDg5ODEyMjc= 4190 Polyfit fails with few non-NaN values 1991007 closed 0     1 2020-07-01T13:26:53Z 2020-08-20T08:34:45Z 2020-08-20T08:34:45Z NONE      

What happened: A linear DataArray.polyfit seems to fail if there are less than 3 non-NaN elements along the fitting dimension.

Traceback ``` TypeError: only size-1 arrays can be converted to Python scalars The above exception was the direct cause of the following exception: Traceback (most recent call last): File "polyfit.py", line 6, in <module> out = arr.polyfit(dim='x', deg=1) File "/home/stephan/venv/variogram/lib/python3.8/site-packages/xarray/core/dataarray.py", line 3455, in polyfit return self._to_temp_dataset().polyfit( File "/home/stephan/venv/variogram/lib/python3.8/site-packages/xarray/core/dataset.py", line 5962, in polyfit coeffs, residuals = duck_array_ops.least_squares( File "/home/stephan/venv/variogram/lib/python3.8/site-packages/xarray/core/duck_array_ops.py", line 625, in least_squares return nputils.least_squares(lhs, rhs, rcond=rcond, skipna=skipna) File "/home/stephan/venv/variogram/lib/python3.8/site-packages/xarray/core/nputils.py", line 239, in least_squares out[:, nan_cols] = np.apply_along_axis( File "<__array_function__ internals>", line 5, in apply_along_axis File "/home/stephan/venv/variogram/lib/python3.8/site-packages/numpy/lib/shape_base.py", line 379, in apply_along_axis res = asanyarray(func1d(inarr_view[ind0], *args, **kwargs)) File "/home/stephan/venv/variogram/lib/python3.8/site-packages/xarray/core/nputils.py", line 227, in _nanpolyfit_1d out[:-1], out[-1], _, _ = np.linalg.lstsq(x[~mask, :], arr[~mask], rcond=rcond) ValueError: setting an array element with a sequence. ```

I've played around with the degree a little bit and the error seems to occur as soon as (# of non-NaN values - degree) < 2

What you expected to happen:

The fit to succeed - I think two non-NaN values should be enough for a linear fit. I also noticed that there is no RankWarning: Polyfit may be poorly conditioned if the degree is larger than the number of non-NaN values.

Minimal Complete Verifiable Example:

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

arr = xr.DataArray([np.nan, 1, 2], dims='x', coords={'x': [0, 1, 2]}) arr.polyfit(dim='x', deg=1) ```

Anything else we need to know?:

Environment:

Output of <tt>xr.show_versions()</tt> INSTALLED VERSIONS ------------------ commit: None python: 3.8.2 (default, Apr 27 2020, 15:53:34) [GCC 9.3.0] python-bits: 64 OS: Linux OS-release: 5.4.0-39-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.4 libnetcdf: 4.6.3 xarray: 0.15.2.dev112+g54b9450b pandas: 1.0.5 numpy: 1.19.0 scipy: 1.5.0 netCDF4: 1.5.3 pydap: None h5netcdf: None h5py: None Nio: None zarr: 2.4.0 cftime: 1.1.3 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2.19.0 distributed: 2.19.0 matplotlib: 3.2.2 cartopy: None seaborn: None numbagg: None pint: None setuptools: 44.0.0 pip: 20.0.2 conda: None pytest: None IPython: 7.16.1 sphinx: None 
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