Match list insert before start in BarrelList

Author: CodingFeng101

boltons/listutils.py

@@ -139,7 +139,7 @@ def insert(self, index, item):
         else:
             list_idx, rel_idx = self._translate_index(index)
             if list_idx is None:
-                raise IndexError()
+                list_idx, rel_idx = 0, 0
             self.lists[list_idx].insert(rel_idx, item)
             self._balance_list(list_idx)
         return

tests/test_listutils.py

@@ -51,6 +51,17 @@ def test_barrel_list():
     bl3[:20:2] = range(0, -10, -1)
     assert bl3[6] == -3  # some truly tricky stepping/slicing works
 
+
+def test_barrel_list_insert_before_start_matches_list():
+    bl = BarrelList(range(int(1e5)))
+    bl._balance_list(0)
+
+    bl.insert(-int(1e9), 'start')
+
+    assert bl[0] == 'start'
+    assert len(bl) == int(1e5) + 1
+
+
 def test_sort_barrel_list():
     bl = BarrelList(reversed(range(100000)))
     bl.pop(50000)