diff --git a/elastica/modules/feature_group.py b/elastica/modules/feature_group.py
index 9578f954..55970a14 100644
--- a/elastica/modules/feature_group.py
+++ b/elastica/modules/feature_group.py
@@ -6,21 +6,50 @@
 
 
 class FeatureGroupFIFO(Iterable):
+    """
+    A class to store the features and their corresponding operators in a FIFO manner.
+
+    Examples
+    --------
+    >>> feature_group = FeatureGroupFIFO()
+    >>> feature_group.append_id(obj_1)
+    >>> feature_group.append_id(obj_2)
+    >>> feature_group.add_operators(obj_1, [OperatorType.ADD, OperatorType.SUBTRACT])
+    >>> feature_group.add_operators(obj_2, [OperatorType.SUBTRACT, OperatorType.MULTIPLY])
+    >>> list(feature_group)
+    [OperatorType.ADD, OperatorType.SUBTRACT, OperatorType.SUBTRACT, OperatorType.MULTIPLY]
+
+    Attributes
+    ----------
+    _operator_collection : list[list[OperatorType]]
+        A list of lists of operators. Each list of operators corresponds to a feature.
+    _operator_ids : list[int]
+        A list of ids of the features.
+
+    Methods
+    -------
+    append_id(feature)
+        Appends the id of the feature to the list of ids.
+    add_operators(feature, operators)
+        Adds the operators to the list of operators corresponding to the feature.
+    """
+
     def __init__(self):
         self._operator_collection: list[list[OperatorType]] = []
         self._operator_ids: list[int] = []
 
     def __iter__(self) -> OperatorType:
-        if not self._operator_collection:
-            raise RuntimeError("Feature group is not instantiated.")
+        """Returns an operator iterator to satisfy the Iterable protocol."""
         operator_chain = itertools.chain.from_iterable(self._operator_collection)
         for operator in operator_chain:
             yield operator
 
     def append_id(self, feature):
+        """Appends the id of the feature to the list of ids."""
         self._operator_ids.append(id(feature))
         self._operator_collection.append([])
 
     def add_operators(self, feature, operators: list[OperatorType]):
+        """Adds the operators to the list of operators corresponding to the feature."""
         idx = self._operator_ids.index(id(feature))
         self._operator_collection[idx].extend(operators)
diff --git a/tests/test_modules/test_feature_grouping.py b/tests/test_modules/test_feature_grouping.py
new file mode 100644
index 00000000..04c0d521
--- /dev/null
+++ b/tests/test_modules/test_feature_grouping.py
@@ -0,0 +1,58 @@
+import pytest
+
+from elastica.modules.feature_group import FeatureGroupFIFO
+
+
+def test_add_ids():
+    feature_group = FeatureGroupFIFO()
+    feature_group.append_id(1)
+    feature_group.append_id(2)
+    feature_group.append_id(3)
+
+    assert feature_group._operator_ids == [id(1), id(2), id(3)]
+
+
+def test_add_operators():
+    feature_group = FeatureGroupFIFO()
+    feature_group.append_id(1)
+    feature_group.add_operators(1, [1, 2, 3])
+    feature_group.append_id(2)
+    feature_group.add_operators(2, [4, 5, 6])
+    feature_group.append_id(3)
+    feature_group.add_operators(3, [7, 8, 9])
+
+    assert feature_group._operator_collection == [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
+    assert feature_group._operator_ids == [id(1), id(2), id(3)]
+
+    feature_group.append_id(4)
+    feature_group.add_operators(4, [10, 11, 12])
+
+    assert feature_group._operator_collection == [
+        [1, 2, 3],
+        [4, 5, 6],
+        [7, 8, 9],
+        [10, 11, 12],
+    ]
+    assert feature_group._operator_ids == [id(1), id(2), id(3), id(4)]
+
+
+def test_grouping():
+    feature_group = FeatureGroupFIFO()
+    feature_group.append_id(1)
+    feature_group.add_operators(1, [1, 2, 3])
+    feature_group.append_id(2)
+    feature_group.add_operators(2, [4, 5, 6])
+    feature_group.append_id(3)
+    feature_group.add_operators(3, [7, 8, 9])
+
+    assert list(feature_group) == [1, 2, 3, 4, 5, 6, 7, 8, 9]
+
+    feature_group.append_id(4)
+    feature_group.add_operators(4, [10, 11, 12])
+
+    assert list(feature_group) == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
+
+    feature_group.append_id(1)
+    feature_group.add_operators(1, [13, 14, 15])
+
+    assert list(feature_group) == [1, 2, 3, 13, 14, 15, 4, 5, 6, 7, 8, 9, 10, 11, 12]