Introduce ToBackend expression

dask_expr/_backends.py

@@ -6,6 +6,7 @@
 from dask.dataframe.backends import DataFrameBackendEntrypoint
 
 from dask_expr._dispatch import get_collection_type
+from dask_expr._expr import ToBackend
 
 try:
     import sparse
@@ -32,25 +33,31 @@
 )
 
 
+class ToPandasBackend(ToBackend):
+    @staticmethod
+    def operation(df, options):
+        from dask.dataframe.dispatch import to_pandas_dispatch
+
+        return to_pandas_dispatch(df, **options)
+
+    def _simplify_down(self):
+        if isinstance(self.frame._meta, (pd.DataFrame, pd.Series, pd.Index)):
+            # We already have pandas data
+            return self.frame
+
+
 class PandasBackendEntrypoint(DataFrameBackendEntrypoint):
     """Pandas-Backend Entrypoint Class for Dask-Expressions
 
     Note that all DataFrame-creation functions are defined
     and registered 'in-place'.
     """
 
-    @classmethod
-    def to_backend_dispatch(cls):
-        from dask.dataframe.dispatch import to_pandas_dispatch
-
-        return to_pandas_dispatch
-
     @classmethod
     def to_backend(cls, data, **kwargs):
-        if isinstance(data._meta, (pd.DataFrame, pd.Series, pd.Index)):
-            # Already a pandas-backed collection
-            return data
-        return data.map_partitions(cls.to_backend_dispatch(), **kwargs)
+        from dask_expr._collection import new_collection
+
+        return new_collection(ToPandasBackend(data, kwargs))
 
 
 dataframe_creation_dispatch.register_backend("pandas", PandasBackendEntrypoint())

dask_expr/_expr.py

@@ -1303,6 +1303,17 @@ def operation(df):
         return df.copy(deep=True)
 
 
+class ToBackend(Elemwise):
+    _parameters = ["frame", "options"]
+    _projection_passthrough = True
+    _filter_passthrough = True
+    _preserves_partitioning_information = True
+
+    @staticmethod
+    def operation(df, options):
+        raise NotImplementedError()
+
+
 class RenameSeries(Elemwise):
     _parameters = ["frame", "index", "sorted_index"]
     _defaults = {"sorted_index": False}

dask_expr/tests/test_collection.py

@@ -2665,3 +2665,12 @@ def test_empty_from_pandas_projection():
     df["foo"] = from_pandas(foo, npartitions=1)
     pdf["foo"] = foo
     assert_eq(df["foo"], pdf["foo"])
+
+
+def test_to_backend_simplify():
+    with dask.config.set({"dataframe.backend": "pandas"}):
+        df = from_dict({"x": [1, 2, 3], "y": [4, 5, 6]}, npartitions=2)
+        df2 = df.to_backend("pandas")[["y"]]
+        assert str(df2.expr) != str(df[["y"]].expr)
+        df3 = df2.simplify()
+        assert str(df3.expr) == str(df[["y"]].expr)