Add support for get_group in GroupBy (#9070)

This PR adds `get_group` functionality to `GroupBy`.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

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

URL: #9070

python/cudf/cudf/core/groupby/groupby.py

@@ -103,6 +103,43 @@ def groups(self):
             zip(group_names.to_pandas(), grouped_index._split(offsets[1:-1]))
         )
 
+    def get_group(self, name, obj=None):
+        """
+        Construct DataFrame from group with provided name.
+
+        Parameters
+        ----------
+        name : object
+            The name of the group to get as a DataFrame.
+        obj : DataFrame, default None
+            The DataFrame to take the DataFrame out of.  If
+            it is None, the object groupby was called on will
+            be used.
+
+        Returns
+        -------
+        group : same type as obj
+
+        Examples
+        --------
+        >>> import cudf
+        >>> df = cudf.DataFrame({"X": ["A", "B", "A", "B"], "Y": [1, 4, 3, 2]})
+        >>> df
+           X  Y
+        0  A  1
+        1  B  4
+        2  A  3
+        3  B  2
+        >>> df.groupby("X").get_group("A")
+           X  Y
+        0  A  1
+        2  A  3
+        """
+        if obj is None:
+            obj = self.obj
+
+        return obj.loc[self.groups[name]]
+
     def size(self):
         """
         Return the size of each group.

python/cudf/cudf/tests/test_groupby.py

@@ -2114,3 +2114,52 @@ def foo(x):
     expect = make_frame(pd.DataFrame, 100).groupby("x").y.apply(foo)
 
     assert_groupby_results_equal(expect, got)
+
+
+@pytest.mark.parametrize(
+    "pdf, group, name, obj",
+    [
+        (
+            pd.DataFrame({"X": ["A", "B", "A", "B"], "Y": [1, 4, 3, 2]}),
+            "X",
+            "A",
+            None,
+        ),
+        (
+            pd.DataFrame({"X": ["A", "B", "A", "B"], "Y": [1, 4, 3, 2]}),
+            "X",
+            "B",
+            None,
+        ),
+        (
+            pd.DataFrame({"X": ["A", "B", "A", "B"], "Y": [1, 4, 3, 2]}),
+            "X",
+            "A",
+            pd.DataFrame({"a": [1, 2, 4, 5, 10, 11]}),
+        ),
+        (
+            pd.DataFrame({"X": ["A", "B", "A", "B"], "Y": [1, 4, 3, 2]}),
+            "Y",
+            1,
+            pd.DataFrame({"a": [1, 2, 4, 5, 10, 11]}),
+        ),
+        (
+            pd.DataFrame({"X": ["A", "B", "A", "B"], "Y": [1, 4, 3, 2]}),
+            "Y",
+            3,
+            pd.DataFrame({"a": [1, 2, 0, 11]}),
+        ),
+    ],
+)
+def test_groupby_get_group(pdf, group, name, obj):
+    gdf = cudf.from_pandas(pdf)
+
+    if isinstance(obj, pd.DataFrame):
+        gobj = cudf.from_pandas(obj)
+    else:
+        gobj = obj
+
+    expected = pdf.groupby(group).get_group(name=name, obj=obj)
+    actual = gdf.groupby(group).get_group(name=name, obj=gobj)
+
+    assert_groupby_results_equal(expected, actual)