raising out of stock exception [out_of_stock]

Author: hjwp

model.py

@@ -4,10 +4,17 @@
 from typing import Optional, List, Set
 
 
+class OutOfStock(Exception):
+    pass
+
+
 def allocate(line: OrderLine, batches: List[Batch]) -> str:
-    batch = next(b for b in sorted(batches) if b.can_allocate(line))
-    batch.allocate(line)
-    return batch.reference
+    try:
+        batch = next(b for b in sorted(batches) if b.can_allocate(line))
+        batch.allocate(line)
+        return batch.reference
+    except StopIteration:
+        raise OutOfStock(f"Out of stock for sku {line.sku}")
 
 
 @dataclass(frozen=True)