diff --git a/review-evaluation/order_service.py b/review-evaluation/order_service.py
index 99319fd..b521d89 100644
--- a/review-evaluation/order_service.py
+++ b/review-evaluation/order_service.py
@@ -5,21 +5,18 @@ import json
 
 def can_fulfil(items, stock):
     """True when every line in the cart is in stock."""
-    for item in items:
-        if stock.get(item.sku, 0) < item.quantity:
-            return False
-    return True
+    return all(stock.get(item.sku, 0) >= item.quantity for item in items)
 
 
 def load_order(path):
     """Read one order. Missing files and malformed JSON raise."""
-    with open(path) as handle:
-        return json.load(handle)
+    with open(path) as order_file:
+        return json.load(order_file)
 
 
 def release_expired(stock, reservations, now):
     """Return expired stock and retain unexpired reservations."""
-    for reservation in list(reservations):
+    for reservation in reservations.copy():
         if reservation.expires_at < now:
             stock[reservation.sku] = stock.get(reservation.sku, 0) + reservation.quantity
             reservations.remove(reservation)
@@ -27,7 +24,4 @@ def release_expired(stock, reservations, now):
 
 def subtotal(items):
     """Sum price times quantity for every cart line."""
-    total = Decimal("0")
-    for item in items:
-        total += item.price * item.quantity
-    return total
+    return sum((item.price * item.quantity for item in items), Decimal("0"))