From 1f188e18804be140bd461cd0d2cd4dca75b93244 Mon Sep 17 00:00:00 2001
From: Ashwin Srinath <shwina@users.noreply.github.com>
Date: Tue, 20 Apr 2021 12:08:48 -0400
Subject: [PATCH 1/8] Add initial list ravel

---
 python/cudf/cudf/core/column/lists.py | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/python/cudf/cudf/core/column/lists.py b/python/cudf/cudf/core/column/lists.py
index da953df5478..c919b5c1fd8 100644
--- a/python/cudf/cudf/core/column/lists.py
+++ b/python/cudf/cudf/core/column/lists.py
@@ -17,6 +17,7 @@
 from cudf.core.buffer import Buffer
 from cudf.core.column import ColumnBase, as_column, column
 from cudf.core.column.methods import ColumnMethodsMixin
+from cudf.core.dtypes import ListDtype
 from cudf.utils.dtypes import is_list_dtype, is_numerical_dtype
 
 
@@ -451,3 +452,26 @@ def sort_values(
             sort_lists(self._column, ascending, na_position),
             retain_index=not ignore_index,
         )
+
+    def ravel(self):
+        result_dtype = self._column.dtype.element_type
+        if not isinstance(result_dtype, ListDtype):
+            raise ValueError(
+                "Cannot ravel a list column with just 1 level of nesting"
+            )
+
+        self_offsets = self._column.children[0]
+        child_offsets = self._column.children[1].children[0]
+        result_offsets = child_offsets[self_offsets]
+        result_children = (result_offsets, self._column.list().leaves)
+
+        return self._return_or_inplace(
+            ListColumn(
+                self._column.size,
+                self._column.dtype.element_type,
+                self._column.mask,
+                self._column.offset,
+                self._column.null_count,
+                result_children,
+            )
+        )

From ddf2907ddb228ed50bdb00048940c06cc6e3c82e Mon Sep 17 00:00:00 2001
From: Ashwin Srinath <shwina@users.noreply.github.com>
Date: Tue, 27 Apr 2021 08:32:04 -0400
Subject: [PATCH 2/8] Add typing for list methods. Ravel tests.

---
 python/cudf/cudf/core/column/categorical.py |  6 +-
 python/cudf/cudf/core/column/lists.py       | 93 ++++++++++++++-------
 python/cudf/cudf/core/column/methods.py     |  2 +
 python/cudf/cudf/core/column/string.py      |  5 +-
 python/cudf/cudf/tests/test_list.py         | 29 +++++++
 5 files changed, 97 insertions(+), 38 deletions(-)

diff --git a/python/cudf/cudf/core/column/categorical.py b/python/cudf/cudf/core/column/categorical.py
index bb1bf3c5d5c..4b9758b9d9d 100644
--- a/python/cudf/cudf/core/column/categorical.py
+++ b/python/cudf/cudf/core/column/categorical.py
@@ -11,7 +11,6 @@
     Optional,
     Sequence,
     Tuple,
-    Union,
     cast,
 )
 
@@ -26,7 +25,7 @@
 from cudf._typing import ColumnLike, Dtype, ScalarLike
 from cudf.core.buffer import Buffer
 from cudf.core.column import column
-from cudf.core.column.methods import ColumnMethodsMixin
+from cudf.core.column.methods import ColumnMethodsMixin, ParentType
 from cudf.core.dtypes import CategoricalDtype
 from cudf.utils.dtypes import (
     is_categorical_dtype,
@@ -45,9 +44,6 @@
     )
 
 
-ParentType = Union["cudf.Series", "cudf.Index"]
-
-
 class CategoricalAccessor(ColumnMethodsMixin):
     _column: CategoricalColumn
 
diff --git a/python/cudf/cudf/core/column/lists.py b/python/cudf/cudf/core/column/lists.py
index c919b5c1fd8..62787770f87 100644
--- a/python/cudf/cudf/core/column/lists.py
+++ b/python/cudf/cudf/core/column/lists.py
@@ -14,14 +14,17 @@
     extract_element,
     sort_lists,
 )
+from cudf._typing import ColumnLike, ScalarLike
 from cudf.core.buffer import Buffer
 from cudf.core.column import ColumnBase, as_column, column
-from cudf.core.column.methods import ColumnMethodsMixin
+from cudf.core.column.methods import ColumnMethodsMixin, ParentType
 from cudf.core.dtypes import ListDtype
 from cudf.utils.dtypes import is_list_dtype, is_numerical_dtype
 
 
 class ListColumn(ColumnBase):
+    dtype: ListDtype
+
     def __init__(
         self, size, dtype, mask=None, offset=0, null_count=None, children=(),
     ):
@@ -178,14 +181,16 @@ class ListMethods(ColumnMethodsMixin):
     List methods for Series
     """
 
-    def __init__(self, column, parent=None):
+    _column: ListColumn
+
+    def __init__(self, column: ListColumn, parent: ParentType = None):
         if not is_list_dtype(column.dtype):
             raise AttributeError(
                 "Can only use .list accessor with a 'list' dtype"
             )
         super().__init__(column=column, parent=parent)
 
-    def get(self, index):
+    def get(self, index: int) -> ParentType:
         """
         Extract element at the given index from each component
 
@@ -217,10 +222,10 @@ def get(self, index):
         else:
             raise IndexError("list index out of range")
 
-    def contains(self, search_key):
+    def contains(self, search_key: ScalarLike) -> ParentType:
         """
-        Creates a column of bool values indicating whether the specified scalar
-        is an element of each row of a list column.
+        Returns boolean values indicating whether the specified scalar
+        is an element of each row.
 
         Parameters
         ----------
@@ -229,7 +234,7 @@ def contains(self, search_key):
 
         Returns
         -------
-        Column
+        Series or Index
 
         Examples
         --------
@@ -257,14 +262,14 @@ def contains(self, search_key):
             return res
 
     @property
-    def leaves(self):
+    def leaves(self) -> ParentType:
         """
         From a Series of (possibly nested) lists, obtain the elements from
         the innermost lists as a flat Series (one value per row).
 
         Returns
         -------
-        Series
+        Series or Index
 
         Examples
         --------
@@ -285,7 +290,7 @@ def leaves(self):
                 self._column.elements, retain_index=False
             )
 
-    def len(self):
+    def len(self) -> ParentType:
         """
         Computes the length of each element in the Series/Index.
 
@@ -309,18 +314,18 @@ def len(self):
         """
         return self._return_or_inplace(count_elements(self._column))
 
-    def take(self, lists_indices):
+    def take(self, lists_indices: ColumnLike) -> ParentType:
         """
         Collect list elements based on given indices.
 
         Parameters
         ----------
-        lists_indices: List type arrays
+        lists_indices: Series-like of lists
             Specifies what to collect from each row
 
         Returns
         -------
-        ListColumn
+        Series or Index
 
         Examples
         --------
@@ -364,14 +369,14 @@ def take(self, lists_indices):
         else:
             return res
 
-    def unique(self):
+    def unique(self) -> ParentType:
         """
-        Returns unique element for each list in the column, order for each
-        unique element is not guaranteed.
+        Returns the unique elements in each list.
+        The ordering of elements is not guaranteed.
 
         Returns
         -------
-        ListColumn
+        Series or Index
 
         Examples
         --------
@@ -401,12 +406,12 @@ def unique(self):
 
     def sort_values(
         self,
-        ascending=True,
-        inplace=False,
-        kind="quicksort",
-        na_position="last",
-        ignore_index=False,
-    ):
+        ascending: bool = True,
+        inplace: bool = False,
+        kind: str = "quicksort",
+        na_position: str = "last",
+        ignore_index: bool = False,
+    ) -> ParentType:
         """
         Sort each list by the values.
 
@@ -423,7 +428,7 @@ def sort_values(
 
         Returns
         -------
-        ListColumn with each list sorted
+        Series or Index with each list sorted
 
         Notes
         -----
@@ -453,17 +458,47 @@ def sort_values(
             retain_index=not ignore_index,
         )
 
-    def ravel(self):
+    def ravel(self) -> ParentType:
+        """
+        Removes one level of nesting from each row of the list Series.
+
+        Returns
+        -------
+        Series or Index with one level of nesting removed from each row.
+
+        Examples
+        --------
+        >>> s1
+        0      [[1.0, 2.0], [3.0, 4.0, 5.0]]
+        1    [[6.0, nan], [7.0], [8.0, 9.0]]
+        dtype: list
+        >>> s1.list.ravel()
+        0    [1.0, 2.0, 3.0, 4.0, 5.0]
+        1    [6.0, nan, 7.0, 8.0, 9.0]
+        dtype: list
+
+        Null values at the top-level in each row are dropped:
+
+        >>> s2
+        0    [[1.0, 2.0], None, [3.0, 4.0, 5.0]]
+        1        [[6.0, nan], [7.0], [8.0, 9.0]]
+        dtype: list
+        >>> s2.list.ravel()
+        0    [1.0, 2.0, 3.0, 4.0, 5.0]
+        1    [6.0, nan, 7.0, 8.0, 9.0]
+        dtype: list
+        """
         result_dtype = self._column.dtype.element_type
         if not isinstance(result_dtype, ListDtype):
-            raise ValueError(
-                "Cannot ravel a list column with just 1 level of nesting"
-            )
+            return self._return_or_inplace(self._column)
 
         self_offsets = self._column.children[0]
         child_offsets = self._column.children[1].children[0]
         result_offsets = child_offsets[self_offsets]
-        result_children = (result_offsets, self._column.list().leaves)
+        result_children = (
+            result_offsets,
+            self._column.children[1].children[1],
+        )
 
         return self._return_or_inplace(
             ListColumn(
diff --git a/python/cudf/cudf/core/column/methods.py b/python/cudf/cudf/core/column/methods.py
index eec9c2a7860..953da311cc1 100644
--- a/python/cudf/cudf/core/column/methods.py
+++ b/python/cudf/cudf/core/column/methods.py
@@ -11,6 +11,8 @@
 if TYPE_CHECKING:
     from cudf.core.column import ColumnBase
 
+ParentType = Union["cudf.Series", "cudf.Index"]
+
 
 class ColumnMethodsMixin:
     _column: ColumnBase
diff --git a/python/cudf/cudf/core/column/string.py b/python/cudf/cudf/core/column/string.py
index de2df9b50d7..6fd6f7cbeb8 100644
--- a/python/cudf/cudf/core/column/string.py
+++ b/python/cudf/cudf/core/column/string.py
@@ -154,7 +154,7 @@
 from cudf._typing import ColumnLike, Dtype, ScalarLike
 from cudf.core.buffer import Buffer
 from cudf.core.column import column, datetime
-from cudf.core.column.methods import ColumnMethodsMixin
+from cudf.core.column.methods import ColumnMethodsMixin, ParentType
 from cudf.utils import utils
 from cudf.utils.docutils import copy_docstring
 from cudf.utils.dtypes import (
@@ -209,9 +209,6 @@
 }
 
 
-ParentType = Union["cudf.Series", "cudf.Index"]
-
-
 class StringMethods(ColumnMethodsMixin):
     def __init__(self, column, parent=None):
         """
diff --git a/python/cudf/cudf/tests/test_list.py b/python/cudf/cudf/tests/test_list.py
index 9906600304b..985cd71ead2 100644
--- a/python/cudf/cudf/tests/test_list.py
+++ b/python/cudf/cudf/tests/test_list.py
@@ -1,5 +1,6 @@
 # Copyright (c) 2020-2021, NVIDIA CORPORATION.
 import functools
+import itertools
 
 import numpy as np
 import pandas as pd
@@ -315,3 +316,31 @@ def test_contains_null_search_key(data, expect):
     expect = cudf.Series(expect, dtype="bool")
     got = sr.list.contains(cudf.Scalar(cudf.NA, sr.dtype.element_type))
     assert_eq(expect, got)
+
+
+@pytest.mark.parametrize(
+    "row",
+    [
+        [[]],
+        [[1]],
+        [[1, 2]],
+        [[1, 2], [3, 4, 5]],
+        [[1, 2, None], [3, 4, 5]],
+        [[[1, 2], [3, 4]], [[5, 6, 7], [8, 9]]],
+        [[["a", "c", "de", None], None, ["fg"]], [["abc", "de"], None]],
+    ],
+)
+def test_ravel(row):
+    def ravel(row):
+        return list(
+            itertools.chain.from_iterable([x for x in row if x is not None])
+        )
+
+    expect = pd.Series([ravel(row)])
+    got = cudf.Series([row]).list.ravel()
+    assert_eq(expect, got)
+
+
+def test_ravel_no_nesting():
+    s = cudf.Series([[1, 2], [3, 4, 5]])
+    assert_eq(s, s.list.ravel())

From dfd1719ec4e5e570848b5649cf56fb2dc66e7262 Mon Sep 17 00:00:00 2001
From: Ashwin Srinath <shwina@users.noreply.github.com>
Date: Mon, 3 May 2021 10:52:44 -0400
Subject: [PATCH 3/8] Rename ravel -> flatten

---
 python/cudf/cudf/core/column/lists.py |  6 +++---
 python/cudf/cudf/tests/test_list.py   | 12 ++++++------
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/python/cudf/cudf/core/column/lists.py b/python/cudf/cudf/core/column/lists.py
index 62787770f87..aa1b96ba1f0 100644
--- a/python/cudf/cudf/core/column/lists.py
+++ b/python/cudf/cudf/core/column/lists.py
@@ -458,7 +458,7 @@ def sort_values(
             retain_index=not ignore_index,
         )
 
-    def ravel(self) -> ParentType:
+    def flatten(self) -> ParentType:
         """
         Removes one level of nesting from each row of the list Series.
 
@@ -472,7 +472,7 @@ def ravel(self) -> ParentType:
         0      [[1.0, 2.0], [3.0, 4.0, 5.0]]
         1    [[6.0, nan], [7.0], [8.0, 9.0]]
         dtype: list
-        >>> s1.list.ravel()
+        >>> s1.list.flatten()
         0    [1.0, 2.0, 3.0, 4.0, 5.0]
         1    [6.0, nan, 7.0, 8.0, 9.0]
         dtype: list
@@ -483,7 +483,7 @@ def ravel(self) -> ParentType:
         0    [[1.0, 2.0], None, [3.0, 4.0, 5.0]]
         1        [[6.0, nan], [7.0], [8.0, 9.0]]
         dtype: list
-        >>> s2.list.ravel()
+        >>> s2.list.flatten()
         0    [1.0, 2.0, 3.0, 4.0, 5.0]
         1    [6.0, nan, 7.0, 8.0, 9.0]
         dtype: list
diff --git a/python/cudf/cudf/tests/test_list.py b/python/cudf/cudf/tests/test_list.py
index 985cd71ead2..37e8934fbff 100644
--- a/python/cudf/cudf/tests/test_list.py
+++ b/python/cudf/cudf/tests/test_list.py
@@ -330,17 +330,17 @@ def test_contains_null_search_key(data, expect):
         [[["a", "c", "de", None], None, ["fg"]], [["abc", "de"], None]],
     ],
 )
-def test_ravel(row):
-    def ravel(row):
+def test_flatten(row):
+    def flatten(row):
         return list(
             itertools.chain.from_iterable([x for x in row if x is not None])
         )
 
-    expect = pd.Series([ravel(row)])
-    got = cudf.Series([row]).list.ravel()
+    expect = pd.Series([flatten(row)])
+    got = cudf.Series([row]).list.flatten()
     assert_eq(expect, got)
 
 
-def test_ravel_no_nesting():
+def test_flatten_no_nesting():
     s = cudf.Series([[1, 2], [3, 4, 5]])
-    assert_eq(s, s.list.ravel())
+    assert_eq(s, s.list.flatten())

From f3ddaba49693979ca1ff19967e775b3da6919676 Mon Sep 17 00:00:00 2001
From: Ashwin Srinath <shwina@users.noreply.github.com>
Date: Mon, 3 May 2021 10:57:18 -0400
Subject: [PATCH 4/8] Don't access children by index

---
 python/cudf/cudf/core/column/lists.py | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/python/cudf/cudf/core/column/lists.py b/python/cudf/cudf/core/column/lists.py
index aa1b96ba1f0..9b686b5a8ca 100644
--- a/python/cudf/cudf/core/column/lists.py
+++ b/python/cudf/cudf/core/column/lists.py
@@ -492,13 +492,10 @@ def flatten(self) -> ParentType:
         if not isinstance(result_dtype, ListDtype):
             return self._return_or_inplace(self._column)
 
-        self_offsets = self._column.children[0]
-        child_offsets = self._column.children[1].children[0]
+        self_offsets = self._column.offsets
+        child_offsets = self._column.elements.offsets
         result_offsets = child_offsets[self_offsets]
-        result_children = (
-            result_offsets,
-            self._column.children[1].children[1],
-        )
+        result_children = (result_offsets, self._column.elements.elements)
 
         return self._return_or_inplace(
             ListColumn(

From 65cfa914503cdb5575d26d33c4d7ed1e70541764 Mon Sep 17 00:00:00 2001
From: Ashwin Srinath <shwina@users.noreply.github.com>
Date: Wed, 26 May 2021 18:01:17 -0400
Subject: [PATCH 5/8] Add bindings for lists::concatenate_list_elements

---
 python/cudf/cudf/_lib/cpp/lists/combine.pxd | 16 +++++++
 python/cudf/cudf/_lib/lists.pyx             | 22 ++++++++-
 python/cudf/cudf/core/column/lists.py       | 51 ++++++++++-----------
 python/cudf/cudf/tests/test_list.py         | 24 ++++------
 4 files changed, 72 insertions(+), 41 deletions(-)

diff --git a/python/cudf/cudf/_lib/cpp/lists/combine.pxd b/python/cudf/cudf/_lib/cpp/lists/combine.pxd
index ea9ade178e2..164253e39b5 100644
--- a/python/cudf/cudf/_lib/cpp/lists/combine.pxd
+++ b/python/cudf/cudf/_lib/cpp/lists/combine.pxd
@@ -3,10 +3,26 @@
 from libcpp.memory cimport unique_ptr
 
 from cudf._lib.cpp.column.column cimport column
+from cudf._lib.cpp.column.column_view cimport column_view
 from cudf._lib.cpp.table.table_view cimport table_view
 
 cdef extern from "cudf/lists/combine.hpp" namespace \
         "cudf::lists" nogil:
+
+    ctypedef enum concatenate_null_policy:
+        IGNORE "cudf::lists::concatenate_null_policy::IGNORE"
+        NULLIFY_OUTPUT_ROW \
+            "cudf::lists::concatenate_null_policy::NULLIFY_OUTPUT_ROW"
+
     cdef unique_ptr[column] concatenate_rows(
         const table_view input_table
     ) except +
+
+    cdef unique_ptr[column] concatenate_list_elements(
+        const table_view input_table,
+    ) except +
+
+    cdef unique_ptr[column] concatenate_list_elements(
+        const column_view input_table,
+        concatenate_null_policy null_policy
+    ) except +
diff --git a/python/cudf/cudf/_lib/lists.pyx b/python/cudf/cudf/_lib/lists.pyx
index 7d8909610dc..9fd7d7611ae 100644
--- a/python/cudf/cudf/_lib/lists.pyx
+++ b/python/cudf/cudf/_lib/lists.pyx
@@ -17,8 +17,11 @@ from cudf._lib.cpp.lists.sorting cimport (
     sort_lists as cpp_sort_lists
 )
 from cudf._lib.cpp.lists.combine cimport (
-    concatenate_rows as cpp_concatenate_rows
+    concatenate_rows as cpp_concatenate_rows,
+    concatenate_null_policy,
+    concatenate_list_elements as cpp_concatenate_list_elements
 )
+
 from cudf._lib.cpp.lists.lists_column_view cimport lists_column_view
 from cudf._lib.cpp.column.column_view cimport column_view
 from cudf._lib.cpp.column.column cimport column
@@ -181,3 +184,20 @@ def concatenate_rows(Table tbl):
 
     result = Column.from_unique_ptr(move(c_result))
     return result
+
+
+def concatenate_list_elements(Column input_column, dropna=False):
+    cdef concatenate_null_policy policy = (
+        concatenate_null_policy.IGNORE if dropna
+        else concatenate_null_policy.NULLIFY_OUTPUT_ROW
+    )
+    cdef column_view c_input = input_column.view()
+    cdef unique_ptr[column] c_result
+
+    with nogil:
+        c_result = move(cpp_concatenate_list_elements(
+            c_input,
+            policy
+        ))
+
+    return Column.from_unique_ptr(move(c_result))
diff --git a/python/cudf/cudf/core/column/lists.py b/python/cudf/cudf/core/column/lists.py
index 219f4a18a5d..2b6badff02b 100644
--- a/python/cudf/cudf/core/column/lists.py
+++ b/python/cudf/cudf/core/column/lists.py
@@ -8,6 +8,7 @@
 import cudf
 from cudf._lib.copying import segmented_gather
 from cudf._lib.lists import (
+    concatenate_list_elements,
     concatenate_rows,
     contains_scalar,
     count_elements,
@@ -518,52 +519,50 @@ def sort_values(
             retain_index=not ignore_index,
         )
 
-    def flatten(self) -> ParentType:
+    def concat(self, dropna=True) -> ParentType:
         """
-        Removes one level of nesting from each row of the list Series.
+        Concatenates the lists in each row
+
+        Parameters
+        ----------
+        dropna: bool,optional
+            If True (default), ignores top-level null elements in each row.
+            If False, and top-level null elements are present, the resulting
+            row in the output is null.
 
         Returns
         -------
-        Series or Index with one level of nesting removed from each row.
+        Series or Index
 
         Examples
         --------
         >>> s1
         0      [[1.0, 2.0], [3.0, 4.0, 5.0]]
-        1    [[6.0, nan], [7.0], [8.0, 9.0]]
+        1    [[6.0, None], [7.0], [8.0, 9.0]]
         dtype: list
-        >>> s1.list.flatten()
+        >>> s1.list.concat()
         0    [1.0, 2.0, 3.0, 4.0, 5.0]
-        1    [6.0, nan, 7.0, 8.0, 9.0]
+        1    [6.0, None, 7.0, 8.0, 9.0]
         dtype: list
 
-        Null values at the top-level in each row are dropped:
+        Null values at the top-level in each row are dropped by default:
 
         >>> s2
         0    [[1.0, 2.0], None, [3.0, 4.0, 5.0]]
-        1        [[6.0, nan], [7.0], [8.0, 9.0]]
+        1        [[6.0, None], [7.0], [8.0, 9.0]]
         dtype: list
-        >>> s2.list.flatten()
+        >>> s2.list.concat()
         0    [1.0, 2.0, 3.0, 4.0, 5.0]
-        1    [6.0, nan, 7.0, 8.0, 9.0]
+        1    [6.0, None, 7.0, 8.0, 9.0]
         dtype: list
-        """
-        result_dtype = self._column.dtype.element_type
-        if not isinstance(result_dtype, ListDtype):
-            return self._return_or_inplace(self._column)
 
-        self_offsets = self._column.offsets
-        child_offsets = self._column.elements.offsets
-        result_offsets = child_offsets[self_offsets]
-        result_children = (result_offsets, self._column.elements.elements)
+        Use ``dropna=False`` to produce a null instead:
 
+        >>> s2.list.concat(dropna=False)
+        0                         None
+        1    [6.0, nan, 7.0, 8.0, 9.0]
+        dtype: list
+        """
         return self._return_or_inplace(
-            ListColumn(
-                self._column.size,
-                self._column.dtype.element_type,
-                self._column.mask,
-                self._column.offset,
-                self._column.null_count,
-                result_children,
-            )
+            concatenate_list_elements(self._column, dropna=dropna)
         )
diff --git a/python/cudf/cudf/tests/test_list.py b/python/cudf/cudf/tests/test_list.py
index 6f959c97f2e..11e3fcd0b6f 100644
--- a/python/cudf/cudf/tests/test_list.py
+++ b/python/cudf/cudf/tests/test_list.py
@@ -1,6 +1,6 @@
 # Copyright (c) 2020-2021, NVIDIA CORPORATION.
 import functools
-import itertools
+import operator
 
 import numpy as np
 import pandas as pd
@@ -331,20 +331,16 @@ def test_contains_null_search_key(data, expect):
         [[["a", "c", "de", None], None, ["fg"]], [["abc", "de"], None]],
     ],
 )
-def test_flatten(row):
-    def flatten(row):
-        return list(
-            itertools.chain.from_iterable([x for x in row if x is not None])
-        )
-
-    expect = pd.Series([flatten(row)])
-    got = cudf.Series([row]).list.flatten()
-    assert_eq(expect, got)
-
+@pytest.mark.parametrize("dropna", [True, False])
+def test_concat_elements(row, dropna):
+    if not dropna and any(x is None for x in row):
+        result = None
+    else:
+        result = functools.reduce(operator.add, row)
 
-def test_flatten_no_nesting():
-    s = cudf.Series([[1, 2], [3, 4, 5]])
-    assert_eq(s, s.list.flatten())
+    expect = pd.Series([result])
+    got = cudf.Series([row]).list.concat(dropna=dropna)
+    assert_eq(expect, got)
 
 
 def test_concatenate_rows_of_lists():

From 48f694980989a2d0d2280e97660a66ee1b82106b Mon Sep 17 00:00:00 2001
From: Ashwin Srinath <shwina@users.noreply.github.com>
Date: Wed, 26 May 2021 18:14:54 -0400
Subject: [PATCH 6/8] Fix test

---
 python/cudf/cudf/tests/test_list.py | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/python/cudf/cudf/tests/test_list.py b/python/cudf/cudf/tests/test_list.py
index 11e3fcd0b6f..34afd2a5036 100644
--- a/python/cudf/cudf/tests/test_list.py
+++ b/python/cudf/cudf/tests/test_list.py
@@ -326,15 +326,22 @@ def test_contains_null_search_key(data, expect):
         [[1]],
         [[1, 2]],
         [[1, 2], [3, 4, 5]],
+        [[1, 2], [], [3, 4, 5]],
         [[1, 2, None], [3, 4, 5]],
+        [[1, 2, None], None, [3, 4, 5]],
+        [[1, 2, None], None, [], [3, 4, 5]],
         [[[1, 2], [3, 4]], [[5, 6, 7], [8, 9]]],
         [[["a", "c", "de", None], None, ["fg"]], [["abc", "de"], None]],
     ],
 )
 @pytest.mark.parametrize("dropna", [True, False])
 def test_concat_elements(row, dropna):
-    if not dropna and any(x is None for x in row):
-        result = None
+    if any(x is None for x in row):
+        if dropna:
+            row = [x for x in row if x is not None]
+            result = functools.reduce(operator.add, row)
+        else:
+            result = None
     else:
         result = functools.reduce(operator.add, row)
 

From 978807aab98fede136a037b5d881a92734d04462 Mon Sep 17 00:00:00 2001
From: Ashwin Srinath <shwina@users.noreply.github.com>
Date: Wed, 26 May 2021 18:23:14 -0400
Subject: [PATCH 7/8] Test raise case

---
 python/cudf/cudf/core/column/lists.py | 16 ++++++++++++----
 python/cudf/cudf/tests/test_list.py   |  6 ++++++
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/python/cudf/cudf/core/column/lists.py b/python/cudf/cudf/core/column/lists.py
index 2b6badff02b..d217309b976 100644
--- a/python/cudf/cudf/core/column/lists.py
+++ b/python/cudf/cudf/core/column/lists.py
@@ -521,7 +521,8 @@ def sort_values(
 
     def concat(self, dropna=True) -> ParentType:
         """
-        Concatenates the lists in each row
+        For a column with at least one level of nesting, concatenate the
+        lists in each row.
 
         Parameters
         ----------
@@ -563,6 +564,13 @@ def concat(self, dropna=True) -> ParentType:
         1    [6.0, nan, 7.0, 8.0, 9.0]
         dtype: list
         """
-        return self._return_or_inplace(
-            concatenate_list_elements(self._column, dropna=dropna)
-        )
+        try:
+            result = concatenate_list_elements(self._column, dropna=dropna)
+        except RuntimeError as e:
+            if "Rows of the input column must be lists." in str(e):
+                raise ValueError(
+                    "list.concat() can only be called on "
+                    "list columns with at least one level "
+                    "of nesting"
+                )
+        return self._return_or_inplace(result)
diff --git a/python/cudf/cudf/tests/test_list.py b/python/cudf/cudf/tests/test_list.py
index 34afd2a5036..f4bd4dec7b4 100644
--- a/python/cudf/cudf/tests/test_list.py
+++ b/python/cudf/cudf/tests/test_list.py
@@ -350,6 +350,12 @@ def test_concat_elements(row, dropna):
     assert_eq(expect, got)
 
 
+def test_concat_elements_raise():
+    s = cudf.Series([[1, 2, 3]])  # no nesting
+    with pytest.raises(ValueError):
+        s.list.concat()
+
+
 def test_concatenate_rows_of_lists():
     pdf = pd.DataFrame({"val": [["a", "a"], ["b"], ["c"]]})
     gdf = cudf.from_pandas(pdf)

From cdb5f4b7f6ea2b8a91cd449d84548f6f081070ef Mon Sep 17 00:00:00 2001
From: Ashwin Srinath <3190405+shwina@users.noreply.github.com>
Date: Wed, 26 May 2021 18:23:54 -0400
Subject: [PATCH 8/8] Update python/cudf/cudf/core/column/lists.py

Co-authored-by: Nghia Truong <ttnghia@users.noreply.github.com>
---
 python/cudf/cudf/core/column/lists.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/python/cudf/cudf/core/column/lists.py b/python/cudf/cudf/core/column/lists.py
index d217309b976..2eb72ae0c2d 100644
--- a/python/cudf/cudf/core/column/lists.py
+++ b/python/cudf/cudf/core/column/lists.py
@@ -526,7 +526,7 @@ def concat(self, dropna=True) -> ParentType:
 
         Parameters
         ----------
-        dropna: bool,optional
+        dropna: bool, optional
             If True (default), ignores top-level null elements in each row.
             If False, and top-level null elements are present, the resulting
             row in the output is null.