What happened?

Coming from xESMF/issues/286. When passing a DataArray to apply_ufunc with the kwarg keep_attrs = False, the attrs on the input DataArray are dropped.

Documentation states:

‘drop’ or False: empty attrs on returned xarray object.

It does not state that the input argument's attrs will be dropped.

What did you expect to happen?

I expected the input DataArray to keep its attrs when passed to apply_ufunc.

Minimal Complete Verifiable Example

import xarray as xr
import numpy as np

def magnitude(a, b):
    func = lambda x, y: np.sqrt(x ** 2 + y ** 2)
    return xr.apply_ufunc(func, a, b, keep_attrs=False)

array = xr.DataArray([1, 2, 3], coords=[("x", [0.1, 0.2, 0.3], {'axis': 'X'})])

before = array['x'].attrs
magnitude(array, -array)
after = array['x'].attrs

print(before)
print(after)

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

No response

Environment