Also special case pint

Author: max-sixty

xarray/core/dataset.py

@@ -81,7 +81,7 @@
 )
 from .missing import get_clean_interp_index
 from .options import OPTIONS, _get_keep_attrs
-from .pycompat import is_duck_dask_array, sparse_array_type
+from .pycompat import is_duck_dask_array, pint_array_type, sparse_array_type
 from .utils import (
     Default,
     Frozen,
@@ -3849,6 +3849,12 @@ def unstack(
                 or sparse
                 # numpy full_like only added `shape` in 1.17
                 or LooseVersion(np.__version__) < LooseVersion("1.17")
+                # pint doesn't implement `np.full_like` in a way that's
+                # currently compatible.
+                # https://github.com/pydata/xarray/pull/4746#issuecomment-753425173
+                or any(
+                    isinstance(v.data, pint_array_type) for v in self.variables.values()
+                )
             ):
                 result = result._unstack_full_reindex(dim, fill_value, sparse)
             else:

xarray/core/pycompat.py

@@ -35,3 +35,10 @@ def is_duck_dask_array(x):
     cupy_array_type = (cupy.ndarray,)
 except ImportError:  # pragma: no cover
     cupy_array_type = ()
+
+try:
+    import pint
+
+    pint_array_type = (pint.Quantity,)
+except ImportError:  # pragma: no cover
+    pint_array_type = ()