Implement get_group. (#7564)

python-package/xgboost/core.py

@@ -884,10 +884,20 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
 
         Returns
         -------
-        base_margin : float
+        base_margin
         """
         return self.get_float_info('base_margin')
 
+    def get_group(self) -> np.ndarray:
+        """Get the group of the DMatrix.
+
+        Returns
+        -------
+        group
+        """
+        group_ptr = self.get_uint_info("group_ptr")
+        return np.diff(group_ptr)
+
     def num_row(self) -> int:
         """Get the number of rows in the DMatrix.
 

tests/python/test_dmatrix.py

@@ -285,6 +285,10 @@ class TestDMatrix:
         dtrain.get_float_info('base_margin')
         dtrain.get_uint_info('group_ptr')
 
+        group_len = np.array([2, 3, 4])
+        dtrain.set_group(group_len)
+        np.testing.assert_equal(group_len, dtrain.get_group())
+
     def test_qid(self):
         rows = 100
         cols = 10