fix decode for scale/ offset list

doc/whats-new.rst

@@ -75,6 +75,8 @@ Bug fixes
 - Add ``missing_dims`` parameter to transpose (:issue:`4647`, :pull:`4767`). By `Daniel Mesejo <https://github.com/mesejo>`_.
 - Resolve intervals before appending other metadata to labels when plotting (:issue:`4322`, :pull:`4794`).
   By `Justus Magin <https://github.com/keewis>`_.
+- Fix regression when decoding a variable with a ``scale_factor`` and ``add_offset`` given
+  as a list of length one (:issue:`4631`) by `Mathias Hauser <https://github.com/mathause>`_.
 - Expand user directory paths (e.g. ``~/``) in :py:func:`open_mfdataset` and
   :py:meth:`Dataset.to_zarr` (:issue:`4783`, :pull:`4795`).
   By `Julien Seguinot <https://github.com/juseg>`_.

xarray/coding/variables.py

@@ -270,9 +270,9 @@ def decode(self, variable, name=None):
             add_offset = pop_to(attrs, encoding, "add_offset", name=name)
             dtype = _choose_float_dtype(data.dtype, "add_offset" in attrs)
             if np.ndim(scale_factor) > 0:
-                scale_factor = scale_factor.item()
+                scale_factor = np.asarray(scale_factor).item()
             if np.ndim(add_offset) > 0:
-                add_offset = add_offset.item()
+                add_offset = np.asarray(add_offset).item()
             transform = partial(
                 _scale_offset_decoding,
                 scale_factor=scale_factor,

xarray/tests/test_coding.py

@@ -8,7 +8,7 @@
 from xarray.coding import variables
 from xarray.conventions import decode_cf_variable, encode_cf_variable
 
-from . import assert_equal, assert_identical, requires_dask
+from . import assert_allclose, assert_equal, assert_identical, requires_dask
 
 with suppress(ImportError):
     import dask.array as da
@@ -105,3 +105,15 @@ def test_scaling_converts_to_float32(dtype):
     roundtripped = coder.decode(encoded)
     assert_identical(original, roundtripped)
     assert roundtripped.dtype == np.float32
+
+
+@pytest.mark.parametrize("scale_factor", (10, [10]))
+@pytest.mark.parametrize("add_offset", (0.1, [0.1]))
+def test_scaling_offset_as_list(scale_factor, add_offset):
+    # test for #4631
+    encoding = dict(scale_factor=scale_factor, add_offset=add_offset)
+    original = xr.Variable(("x",), np.arange(10.0), encoding=encoding)
+    coder = variables.CFScaleOffsetCoder()
+    encoded = coder.encode(original)
+    roundtripped = coder.decode(encoded)
+    assert_allclose(original, roundtripped)