Add struct.explode() method (#8729)

Part of #8660. Note that the issue is asking for this feature in _dask-cudf_, which this PR does not implement.

Depends on: #8306

Authors:
  - Ashwin Srinath (https://github.com/shwina)

Approvers:
  - https://github.com/brandon-b-miller
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #8729

python/cudf/cudf/core/column/struct.py

@@ -150,6 +150,8 @@ class StructMethods(ColumnMethods):
     Struct methods for Series
     """
 
+    _column: StructColumn
+
     def __init__(self, parent=None):
         if not is_struct_dtype(parent.dtype):
             raise AttributeError(
@@ -190,3 +192,38 @@ def field(self, key):
             return self._return_or_inplace(self._column.children[pos])
         else:
             return self._return_or_inplace(self._column.children[key])
+
+    def explode(self):
+        """
+        Return a DataFrame whose columns are the fields of this struct Series.
+
+        Notes
+        -----
+        Note that a copy of the columns is made.
+
+        Examples
+        --------
+        >>> s
+        0    {'a': 1, 'b': 'x'}
+        1    {'a': 2, 'b': 'y'}
+        2    {'a': 3, 'b': 'z'}
+        3    {'a': 4, 'b': 'a'}
+        dtype: struct
+
+        >>> s.struct.explode()
+           a  b
+        0  1  x
+        1  2  y
+        2  3  z
+        3  4  a
+        """
+        return cudf.DataFrame._from_data(
+            cudf.core.column_accessor.ColumnAccessor(
+                {
+                    name: col.copy(deep=True)
+                    for name, col in zip(
+                        self._column.dtype.fields, self._column.children
+                    )
+                }
+            )
+        )

python/cudf/cudf/core/dataframe.py

@@ -7563,18 +7563,23 @@ def to_dict(self, orient="dict", into=dict):
     def to_struct(self, name=None):
         """
         Return a struct Series composed of the columns of the DataFrame.
-        Note that no copies of the data are made.
 
         Parameters
         ----------
         name: optional
             Name of the resulting Series
+
+        Notes
+        -----
+        Note that a copy of the columns is made.
         """
         col = cudf.core.column.build_struct_column(
             names=self._data.names, children=self._data.columns, size=len(self)
         )
         return cudf.Series._from_data(
-            cudf.core.column_accessor.ColumnAccessor({name: col}),
+            cudf.core.column_accessor.ColumnAccessor(
+                {name: col.copy(deep=True)}
+            ),
             index=self.index,
             name=name,
         )

python/cudf/cudf/tests/test_struct.py

@@ -166,6 +166,28 @@ def test_struct_scalar_null():
     assert slr.device_value.value is cudf.NA
 
 
+def test_struct_explode():
+    s = cudf.Series([], dtype=cudf.StructDtype({}))
+    expect = cudf.DataFrame({})
+    assert_eq(expect, s.struct.explode())
+
+    s = cudf.Series(
+        [
+            {"a": 1, "b": "x"},
+            {"a": 2, "b": "y"},
+            {"a": 3, "b": "z"},
+            {"a": 4, "b": "a"},
+        ]
+    )
+    expect = cudf.DataFrame({"a": [1, 2, 3, 4], "b": ["x", "y", "z", "a"]})
+    got = s.struct.explode()
+    assert_eq(expect, got)
+
+    # check that a copy was made:
+    got["a"][0] = 5
+    assert_eq(s.struct.explode(), expect)
+
+
 def test_dataframe_to_struct():
     df = cudf.DataFrame()
     expect = cudf.Series(dtype=cudf.StructDtype({}))
@@ -179,6 +201,10 @@ def test_dataframe_to_struct():
     got = df.to_struct()
     assert_eq(expect, got)
 
+    # check that a copy was made:
+    df["a"][0] = 5
+    assert_eq(got, expect)
+
 
 @pytest.mark.parametrize(
     "series, start, end",