fix: support negative indices in OrderedSet.__getitem__

statemachine/orderedset.py

@@ -62,10 +62,14 @@ class OrderedSet(MutableSet[T]):
     >>> s[2]
     3
 
-    >>> eval(repr(OrderedSet(['a', 'b', 'c'])))
-    OrderedSet(['a', 'b', 'c'])
+    >>> s[-1]
+    3
 
+    >>> s[-3]
+    1
 
+    >>> eval(repr(OrderedSet(['a', 'b', 'c'])))
+    OrderedSet(['a', 'b', 'c'])
 
     """
 
@@ -84,10 +88,15 @@ def discard(self, x: T) -> None:
         self._d.pop(x, None)
 
     def __getitem__(self, index) -> T:
+        original_index = index
+        if index < 0:
+            index += len(self)
+        if index < 0:
+            raise IndexError(f"index {original_index} out of range")
         try:
             return next(itertools.islice(self._d, index, index + 1))
         except StopIteration as e:
-            raise IndexError(f"index {index} out of range") from e
+            raise IndexError(f"index {original_index} out of range") from e
 
     def __contains__(self, x: object) -> bool:
         return self._d.__contains__(x)

tests/test_state.py

@@ -56,13 +56,28 @@ def test_ordered_set_getitem():
     assert s[2] == 30
 
 
+def test_ordered_set_getitem_negative_index():
+    """OrderedSet supports negative index access like Python sequences."""
+    s = OrderedSet([10, 20, 30])
+    assert s[-1] == 30
+    assert s[-2] == 20
+    assert s[-3] == 10
+
+
 def test_ordered_set_getitem_out_of_range():
     """OrderedSet raises IndexError for out-of-range index."""
     s = OrderedSet([10, 20])
     with pytest.raises(IndexError, match="index 5 out of range"):
         s[5]
 
 
+def test_ordered_set_getitem_negative_out_of_range():
+    """OrderedSet raises IndexError for negative out-of-range index."""
+    s = OrderedSet([10, 20])
+    with pytest.raises(IndexError, match=r"index -3 out of range"):
+        s[-3]
+
+
 def test_ordered_set_union():
     """OrderedSet.union returns new set with elements from both."""
     s1 = OrderedSet([1, 2])