Python API for LIstMethods.len() (#7283)

Closes #7157 

This PR adds `ListMethods.len()` API that returns an integer column that contains the length for each element in a `ListColumn`.
Example:
```python
>>> s = cudf.Series([[1,2], None, [3]])
>>> s
0    [1, 2]
1      None
2       [3]
dtype: list
>>> s.list.len()
0       2
1    <NA>
2       1
dtype: int32
```

Authors:
  - Michael Wang (@isVoid)
  - Ashwin Srinath (@shwina)

Approvers:
  - Keith Kraus (@kkraus14)
  - @brandon-b-miller

URL: #7283

python/cudf/cudf/_lib/cpp/lists/count_elements.pxd

@@ -0,0 +1,9 @@
+# Copyright (c) 2021, NVIDIA CORPORATION.
+
+from libcpp.memory cimport unique_ptr
+
+from cudf._lib.cpp.column.column cimport column
+from cudf._lib.cpp.lists.lists_column_view cimport lists_column_view
+
+cdef extern from "cudf/lists/count_elements.hpp" namespace "cudf::lists" nogil:
+    cdef unique_ptr[column] count_elements(const lists_column_view) except +

python/cudf/cudf/_lib/cpp/lists/lists_column_view.pxd

@@ -7,7 +7,6 @@ from cudf._lib.cpp.column.column_view cimport (
 
 cdef extern from "cudf/lists/lists_column_view.hpp" namespace "cudf" nogil:
     cdef cppclass lists_column_view(column_view):
-        lists_column_view() except +
         lists_column_view(const column_view& lists_column) except +
         column_view parent() except +
         column_view offsets() except +

python/cudf/cudf/_lib/lists.pyx

@@ -0,0 +1,34 @@
+# Copyright (c) 2021, NVIDIA CORPORATION.
+
+from libcpp.memory cimport unique_ptr, shared_ptr, make_shared
+from libcpp.utility cimport move
+
+from cudf._lib.cpp.lists.count_elements cimport (
+    count_elements as cpp_count_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
+
+from cudf._lib.column cimport Column
+
+
+from cudf.core.dtypes import ListDtype
+
+
+def count_elements(Column col):
+    if not isinstance(col.dtype, ListDtype):
+        raise TypeError("col is not a list column.")
+
+    # shared_ptr required because lists_column_view has no default
+    # ctor
+    cdef shared_ptr[lists_column_view] list_view = (
+        make_shared[lists_column_view](col.view())
+    )
+    cdef unique_ptr[column] c_result
+
+    with nogil:
+        c_result = move(cpp_count_elements(list_view.get()[0]))
+
+    result = Column.from_unique_ptr(move(c_result))
+    return result

python/cudf/cudf/core/column/lists.py

@@ -5,6 +5,7 @@
 import pyarrow as pa
 
 import cudf
+from cudf._lib.lists import count_elements
 from cudf.core.buffer import Buffer
 from cudf.core.column import ColumnBase, column
 from cudf.core.column.methods import ColumnMethodsMixin
@@ -203,3 +204,27 @@ def leaves(self):
             return self._return_or_inplace(
                 self._column.elements, retain_index=False
             )
+
+    def len(self):
+        """
+        Computes the length of each element in the Series/Index.
+
+        Returns
+        -------
+        Series or Index
+
+        Examples
+        --------
+        >>> s = cudf.Series([[1, 2, 3], None, [4, 5]])
+        >>> s
+        0    [1, 2, 3]
+        1         None
+        2       [4, 5]
+        dtype: list
+        >>> s.list.len()
+        0       3
+        1    <NA>
+        2       2
+        dtype: int32
+        """
+        return self._return_or_inplace(count_elements(self._column))

python/cudf/cudf/tests/test_list.py

@@ -89,3 +89,26 @@ def test_listdtype_hash():
     c = cudf.core.dtypes.ListDtype("int32")
 
     assert hash(a) != hash(c)
+
+
+@pytest.mark.parametrize(
+    "data",
+    [
+        [[]],
+        [[1, 2, 3], [4, 5]],
+        [[1, 2, 3], [], [4, 5]],
+        [[1, 2, 3], None, [4, 5]],
+        [[None, None], [None]],
+        [[[[[[1, 2, 3]]]]]],
+        cudf.Series([[1, 2]]).iloc[0:0],
+        cudf.Series([None, [1, 2]]).iloc[0:1],
+    ],
+)
+def test_len(data):
+    gsr = cudf.Series(data)
+    psr = gsr.to_pandas()
+
+    expect = psr.map(lambda x: len(x) if x is not None else None)
+    got = gsr.list.len()
+
+    assert_eq(expect, got, check_dtype=False)