Dask friendly check in .weighted()

doc/whats-new.rst

@@ -38,6 +38,8 @@ New Features
   By `Stephan Hoyer <https://github.com/shoyer>`_.
 - Added typehints in :py:func:`align` to reflect that the same type received in ``objects`` arg will be returned (:pull:`4522`).
   By `Michal Baumgartner <https://github.com/m1so>`_.
+- :py:meth:`Dataset.weighted` and :py:meth:`DataArray.weighted` are now executing value checks lazily if weights are provided as dask arrays (:issue:`4541`, :pull:`4559`).
+  By `Julius Busecke <https://github.com/jbusecke>`_.
 
 Bug fixes
 ~~~~~~~~~

xarray/core/weighted.py

@@ -1,7 +1,9 @@
 from typing import TYPE_CHECKING, Hashable, Iterable, Optional, Union, overload
 
+from . import duck_array_ops
 from .computation import dot
 from .options import _get_keep_attrs
+from .pycompat import is_duck_dask_array
 
 if TYPE_CHECKING:
     from .dataarray import DataArray, Dataset
@@ -100,12 +102,24 @@ def __init__(self, obj, weights):
         if not isinstance(weights, DataArray):
             raise ValueError("`weights` must be a DataArray")
 
-        if weights.isnull().any():
-            raise ValueError(
-                "`weights` cannot contain missing values. "
-                "Missing values can be replaced by `weights.fillna(0)`."
+        def _weight_check(w):
+            # Ref https://github.com/pydata/xarray/pull/4559/files#r515968670
+            if duck_array_ops.isnull(w).any():
+                raise ValueError(
+                    "`weights` cannot contain missing values. "
+                    "Missing values can be replaced by `weights.fillna(0)`."
+                )
+            return w
+
+        if is_duck_dask_array(weights.data):
+            # assign to copy - else the check is not triggered
+            weights = weights.copy(
+                data=weights.data.map_blocks(_weight_check, dtype=weights.dtype)
             )
 
+        else:
+            _weight_check(weights.data)
+
         self.obj = obj
         self.weights = weights
 

xarray/tests/test_weighted.py

@@ -5,6 +5,8 @@
 from xarray import DataArray
 from xarray.tests import assert_allclose, assert_equal, raises_regex
 
+from . import raise_if_dask_computes, requires_dask
+
 
 @pytest.mark.parametrize("as_dataset", (True, False))
 def test_weighted_non_DataArray_weights(as_dataset):
@@ -29,6 +31,24 @@ def test_weighted_weights_nan_raises(as_dataset, weights):
         data.weighted(DataArray(weights))
 
 
+@requires_dask
+@pytest.mark.parametrize("as_dataset", (True, False))
+@pytest.mark.parametrize("weights", ([np.nan, 2], [np.nan, np.nan]))
+def test_weighted_weights_nan_raises_dask(as_dataset, weights):
+
+    data = DataArray([1, 2]).chunk({"dim_0": -1})
+    if as_dataset:
+        data = data.to_dataset(name="data")
+
+    weights = DataArray(weights).chunk({"dim_0": -1})
+
+    with raise_if_dask_computes():
+        weighted = data.weighted(weights)
+
+    with pytest.raises(ValueError, match="`weights` cannot contain missing values."):
+        weighted.sum().load()
+
+
 @pytest.mark.parametrize(
     ("weights", "expected"),
     (([1, 2], 3), ([2, 0], 2), ([0, 0], np.nan), ([-1, 1], np.nan)),