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cjauvin · cjauvin · commit 4b7d866ab271 · 2021-10-15T16:23:16.000-04:00
diff --git a/xarray/core/weighted.py b/xarray/core/weighted.py
@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Generic, Hashable, Iterable, Optional, Union
+from typing import TYPE_CHECKING, Generic, Hashable, Iterable, Optional, Union, cast
 
 import numpy as np
 
@@ -237,7 +237,7 @@ def _weighted_std(
     ) -> "DataArray":
         """Reduce a DataArray by a weighted ``std`` along some dimension(s)."""
 
-        return np.sqrt(self._weighted_var(da, dim, skipna))
+        return cast("DataArray", np.sqrt(self._weighted_var(da, dim, skipna)))
 
     def _implementation(self, func, dim, **kwargs):
 
diff --git a/xarray/tests/test_weighted.py b/xarray/tests/test_weighted.py
@@ -503,7 +503,9 @@ def test_weighted_operations_different_shapes(
     check_weighted_operations(data, weights, None, skipna)
 
 
-@pytest.mark.parametrize("operation", ("sum_of_weights", "sum", "mean", "sum_of_squares", "var", "std"))
+@pytest.mark.parametrize(
+    "operation", ("sum_of_weights", "sum", "mean", "sum_of_squares", "var", "std")
+)
 @pytest.mark.parametrize("as_dataset", (True, False))
 @pytest.mark.parametrize("keep_attrs", (True, False, None))
 def test_weighted_operations_keep_attr(operation, as_dataset, keep_attrs):
@@ -530,7 +532,9 @@ def test_weighted_operations_keep_attr(operation, as_dataset, keep_attrs):
     assert not result.attrs
 
 
-@pytest.mark.parametrize("operation", ("sum_of_weights", "sum", "mean", "sum_of_squares", "var", "std"))
+@pytest.mark.parametrize(
+    "operation", ("sum_of_weights", "sum", "mean", "sum_of_squares", "var", "std")
+)
 def test_weighted_operations_keep_attr_da_in_ds(operation):
     # GH #3595
 
