allow other and drop arguments in where (gh#6466)

doc/whats-new.rst

@@ -78,6 +78,9 @@ Bug fixes
   By `Stan West <https://github.com/stanwest>`_.
 - Fix bug in :py:func:`where` when passing non-xarray objects with ``keep_attrs=True``. (:issue:`6444`, :pull:`6461`)
   By `Sam Levang <https://github.com/slevang>`_.
+- Allow passing both ``other`` and ``drop=True`` arguments to ``xr.DataArray.where``
+  and ``xr.Dataset.where`` (:pull:`6466`, :pull:`6467`).
+  By `Michael Delgado <https://github.com/delgadom>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/common.py

@@ -1197,8 +1197,7 @@ def where(self, cond, other=dtypes.NA, drop: bool = False):
             By default, these locations filled with NA.
         drop : bool, optional
             If True, coordinate labels that only correspond to False values of
-            the condition are dropped from the result. Mutually exclusive with
-            ``other``.
+            the condition are dropped from the result.
 
         Returns
         -------
@@ -1251,6 +1250,14 @@ def where(self, cond, other=dtypes.NA, drop: bool = False):
                [15., nan, nan, nan]])
         Dimensions without coordinates: x, y
 
+        >>> a.where(a.x + a.y < 4, -1, drop=True)
+        <xarray.DataArray (x: 4, y: 4)>
+        array([[ 0,  1,  2,  3],
+               [ 5,  6,  7, -1],
+               [10, 11, -1, -1],
+               [15, -1, -1, -1]])
+        Dimensions without coordinates: x, y
+
         See Also
         --------
         numpy.where : corresponding numpy function
@@ -1264,9 +1271,6 @@ def where(self, cond, other=dtypes.NA, drop: bool = False):
             cond = cond(self)
 
         if drop:
-            if other is not dtypes.NA:
-                raise ValueError("cannot set `other` if drop=True")
-
             if not isinstance(cond, (Dataset, DataArray)):
                 raise TypeError(
                     f"cond argument is {cond!r} but must be a {Dataset!r} or {DataArray!r}"

xarray/tests/test_dataset.py

@@ -4548,8 +4548,11 @@ def test_where_other(self):
         actual = ds.where(lambda x: x > 1, -1)
         assert_equal(expected, actual)
 
-        with pytest.raises(ValueError, match=r"cannot set"):
-            ds.where(ds > 1, other=0, drop=True)
+        actual = ds.where(ds > 1, other=-1, drop=True)
+        expected_nodrop = ds.where(ds > 1, -1)
+        _, expected = xr.align(actual, expected_nodrop, join="left")
+        assert_equal(actual, expected)
+        assert actual.a.dtype == int
 
         with pytest.raises(ValueError, match=r"cannot align .* are not equal"):
             ds.where(ds > 1, ds.isel(x=slice(3)))