Items added to one cart show up in other carts

Usage by agent

Timeline

1. planner iteration 1 ✓Finished 3 calls · 7,262 tokens · $0.0006 · 6.8s

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2. coder iteration 1 ✓Finished 3 calls · 11.1K tokens · $0.0007 · 5.8s

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3. tester iteration 1 ✓Finished 3 calls · 13.5K tokens · $0.0009 · 9.8s

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4. verify iteration 1 ✓Passed 0.5s

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   6 passed in 0.01s
   

Final diff

diff --git a/cart.py b/cart.py
index bff9b5d..9d4c165 100644
--- a/cart.py
+++ b/cart.py
@@ -1,6 +1,6 @@
 class Cart:
-    def __init__(self, items=[]):
-        self.items = items
+    def __init__(self, items=None):
+        self.items = list(items) if items is not None else []
 
     def add(self, item: str) -> None:
         self.items.append(item)
diff --git a/test_cart.py b/test_cart.py
index 2040527..e568f8b 100644
--- a/test_cart.py
+++ b/test_cart.py
@@ -5,3 +5,36 @@ def test_add_and_count():
     cart = Cart(items=["a"])
     cart.add("b")
     assert cart.count() == 2
+
+
+def test_carts_do_not_share_items():
+    first = Cart()
+    first.add("x")
+    second = Cart()
+    assert second.count() == 0
+    assert first.count() == 1
+
+
+def test_passed_list_is_not_mutated():
+    original = ["a"]
+    cart = Cart(items=original)
+    cart.add("b")
+    assert original == ["a"]
+    assert cart.count() == 2
+
+
+def test_new_cart_is_empty():
+    assert Cart().count() == 0
+
+
+def test_default_carts_have_distinct_item_lists():
+    first = Cart()
+    second = Cart()
+    assert first.items is not second.items
+
+
+def test_tuple_items_are_accepted():
+    cart = Cart(items=("a",))
+    cart.add("b")
+    assert cart.items == ["a", "b"]
+    assert cart.count() == 2