Reimplementation of NestedFrame.reduce()

src/nested_pandas/nestedframe/core.py

@@ -413,35 +413,15 @@ def my_sum(col1, col2):
         if len(requested_columns) < len(args):
             extra_args = args[len(requested_columns) :]
 
-        # find targeted layers
-        layers = np.unique([col[0] for col in requested_columns])
-
-        # build a flat dataframe with array columns to apply to the function
-        apply_df = NestedFrame()
-        for layer in layers:
+        iterators = []
+        for layer, col in requested_columns:
             if layer == "base":
-                columns = [col[1] for col in requested_columns if col[0] == layer]
-                apply_df = apply_df.join(self[columns], how="outer")
+                iterators.append(self[col])
             else:
-                # TODO: It should be faster to pass these columns to to_lists, but its 20x slower
-                # columns = [col[1] for col in requested_columns if col[0] == layer]
-                apply_df = apply_df.join(self[layer].nest.to_lists(), how="outer")
+                iterators.append(self[layer].array.iter_field_lists(col))
 
-        # Translates the requested columns into the scalars or arrays we pass to func.
-        def translate_cols(frame, layer, col):
-            if layer == "base":
-                # We pass the "base" column as a scalar
-                return frame[col]
-            return np.asarray(frame[col])
-
-        # send arrays along to the apply call
-        result = apply_df.apply(
-            lambda x: func(
-                *[translate_cols(x, layer, col) for layer, col in requested_columns], *extra_args, **kwargs
-            ),
-            axis=1,  # to apply func on each row of our nested frame)
-        )
-        return result
+        results = [func(*cols, *extra_args, **kwargs) for cols in zip(*iterators)]
+        return NestedFrame(results, index=self.index)
 
     def to_parquet(self, path, by_layer=False, **kwargs) -> None:
         """Creates parquet file(s) with the data of a NestedFrame, either

src/nested_pandas/series/ext_array.py

@@ -35,7 +35,7 @@
 # typing.Self and "|" union syntax don't exist in Python 3.9
 from __future__ import annotations
 
-from collections.abc import Iterable, Iterator, Sequence
+from collections.abc import Generator, Iterable, Iterator, Sequence
 from typing import Any, Callable, cast
 
 import numpy as np
@@ -648,8 +648,27 @@ def num_chunks(self) -> int:
         """Number of chunks in underlying pyarrow.ChunkedArray"""
         return self._chunked_array.num_chunks
 
+    def iter_field_lists(self, field: str) -> Generator[np.ndarray, None, None]:
+        """Iterate over single field nested lists, as numpy arrays
+
+        Parameters
+        ----------
+        field : str
+            The name of the field to iterate over.
+
+        Yields
+        ------
+        np.ndarray
+            The numpy array view over a list scalar.
+        """
+        for chunk in self._chunked_array.iterchunks():
+            struct_array: pa.StructArray = cast(pa.StructArray, chunk)
+            list_array: pa.ListArray = cast(pa.ListArray, struct_array.field(field))
+            for list_scalar in list_array:
+                yield np.asarray(list_scalar.values)
+
     def view_fields(self, fields: str | list[str]) -> Self:  # type: ignore[name-defined] # noqa: F821
-        """Get a view of the series with only the specified fields
+        """Get a view of the extension array with only the specified fields
 
         Parameters
         ----------
@@ -659,7 +678,7 @@ def view_fields(self, fields: str | list[str]) -> Self:  # type: ignore[name-def
         Returns
         -------
         NestedExtensionArray
-            The view of the series with only the specified fields.
+            The view of the array with only the specified fields.
         """
         if isinstance(fields, str):
             fields = [fields]

tests/nested_pandas/series/test_ext_array.py

@@ -1255,6 +1255,22 @@ def test_num_chunks():
     assert ext_array.num_chunks == 7
 
 
+def test_iter_field_lists():
+    """Test .iter_field_lists() yields the correct field lists"""
+    a = [[1, 2, 3], [1, 2, 3, 4]]
+    b = [np.array(["a", "b", "c"]), np.array(["x", "y", "z", "w"])]
+    struct_array = pa.StructArray.from_arrays(
+        arrays=[a, b],
+        names=["a", "b"],
+    )
+    ext_array = NestedExtensionArray(struct_array)
+
+    for actual, desired in zip(ext_array.iter_field_lists("a"), a):
+        assert_array_equal(actual, desired)
+    for actual, desired in zip(ext_array.iter_field_lists("b"), b):
+        assert_array_equal(actual, desired)
+
+
 def test_view_fields_with_single_field():
     """Tests ext_array.view("field")"""
     arrays = [