Add how="leftanti" support for cudf-backed merge

dask_expr/_collection.py

@@ -2818,7 +2818,7 @@ def merge(
         Parameters
         ----------
         right: dask.dataframe.DataFrame
-        how : {'left', 'right', 'outer', 'inner', 'leftsemi'}, default: 'inner'
+        how : {'left', 'right', 'outer', 'inner', 'leftsemi', 'leftanti'}, default: 'inner'
             How to handle the operation of the two objects:
 
             - left: use calling frame's index (or column if on is specified)
@@ -2830,6 +2830,9 @@ def merge(
               on is specified) with other frame's index, preserving the order
               of the calling's one
             - leftsemi: Choose all rows in left where the join keys can be found
+              in right. Won't duplicate rows if the keys are duplicated in right.
+              Drops all columns from right. Only supported with 'cudf' backend.
+            - leftanti: Choose all rows in left where the join keys cannot be found
               in right. Won't duplicate rows if the keys are duplicated in right.
               Drops all columns from right.
 
@@ -5628,11 +5631,12 @@ def merge(
     if on and not left_on and not right_on:
         left_on = right_on = on
 
-    supported_how = ("left", "right", "outer", "inner", "leftsemi")
+    supported_how = ("left", "right", "outer", "inner", "leftsemi", "leftanti")
     if how not in supported_how:
         raise ValueError(
             f"dask.dataframe.merge does not support how='{how}'."
             f"Options are: {supported_how}."
+            "Note that 'leftanti' is only an dask_cudf option."
         )
 
     if how == "leftsemi":

dask_expr/_merge.py

@@ -102,7 +102,7 @@ def _filter_passthrough_available(self, parent, dependents):
             if predicate_columns is None:
                 return False
             if predicate_columns.issubset(self.left.columns):
-                return self.how in ("left", "inner", "leftsemi")
+                return self.how in ("left", "inner", "leftsemi", "leftanti")
             elif predicate_columns.issubset(self.right.columns):
                 return self.how in ("right", "inner")
             elif len(predicate_columns) > 0:
@@ -242,7 +242,7 @@ def _divisions(self):
             elif (
                 use_left
                 and self.right.npartitions == 1
-                and self.how in ("inner", "left", "leftsemi")
+                and self.how in ("inner", "left", "leftsemi", "leftanti")
             ):
                 return self.left.divisions
             else:
@@ -283,7 +283,7 @@ def is_broadcast_join(self):
         s_method = self.shuffle_method or get_default_shuffle_method()
         if (
             s_method in ("tasks", "p2p")
-            and self.how in ("inner", "left", "right", "leftsemi")
+            and self.how in ("inner", "left", "right", "leftsemi", "leftanti")
             and self.how != broadcast_side
             and broadcast is not False
         ):
@@ -301,7 +301,7 @@ def _is_single_partition_broadcast(self):
             or self.left.npartitions == 1
             and self.how in ("right", "inner")
             or self.right.npartitions == 1
-            and self.how in ("left", "inner", "leftsemi")
+            and self.how in ("left", "inner", "leftsemi", "leftanti")
         )
 
     @functools.cached_property
@@ -776,7 +776,7 @@ def _layer(self) -> dict:
                     ),
                     (bcast_name, j),
                 ]
-                if self.broadcast_side in ("left", "leftsemi"):
+                if self.broadcast_side in ("left", "leftsemi", "leftanti"):
                     _merge_args.reverse()
 
                 inter_key = (inter_name, part_out, j)

dask_expr/tests/test_merge.py

@@ -8,7 +8,7 @@
 from dask_expr._merge import BroadcastJoin
 from dask_expr._shuffle import Shuffle
 from dask_expr.io import FromPandas
-from dask_expr.tests._util import _backend_library, assert_eq
+from dask_expr.tests._util import _backend_library, _backend_name, assert_eq
 
 # Set DataFrame backend for this module
 pd = _backend_library()
@@ -996,6 +996,33 @@ def test_merge_leftsemi():
         df1.merge(df2, how="leftsemi", on="aa")
 
 
+@pytest.mark.xfail(
+    _backend_name() != "cudf", reason="leftanti joins not supported with pandas backend"
+)
+def test_merge_leftanti_cudf():
+    pdf1 = pd.DataFrame({"aa": [1, 2, 3, 4, 5, 6, 1, 2, 3], "bb": 1})
+    pdf2 = pd.DataFrame({"aa": [1, 2, 2, 4, 4, 10], "cc": 1})
+
+    df1 = from_pandas(pdf1, npartitions=2)
+    df2 = from_pandas(pdf2, npartitions=2)
+    assert_eq(
+        df1.merge(df2, how="leftanti"),
+        pdf1[~pdf1.aa.isin(pdf2.aa)],
+        check_index=False,
+    )
+    df2 = df2.rename(columns={"aa": "dd"})
+    assert_eq(
+        df1.merge(df2, how="leftanti", left_on="aa", right_on="dd"),
+        pdf1[~pdf1.aa.isin(pdf2.aa)],
+        check_index=False,
+    )
+    assert_eq(df1.merge(df2, how="leftanti"), pdf1[~pdf1.index.isin(pdf2.index)])
+
+    pdf2 = pdf2.set_index("aa")
+    df2 = from_pandas(pdf2, npartitions=2)
+    assert_eq(df1.merge(df2, how="leftanti", on="aa"), pdf1[~pdf1.aa.isin(pdf2.index)])
+
+
 def test_merge_suffix_projections():
     df = pd.DataFrame(
         {