Port Round 1 to Python (OrderDomain): 7 tests green, START state preserved

python/src/order_domain/__init__.py

@@ -0,0 +1,3 @@
+from order_domain.order import Order, OrderLine
+
+__all__ = ["Order", "OrderLine"]

python/src/order_domain/order.py

@@ -0,0 +1,39 @@
+from decimal import Decimal
+
+
+class OrderLine:
+    def __init__(self, unit_price: Decimal, quantity: int):
+        self.unit_price = unit_price
+        self.quantity = quantity
+        self.has_discount = False
+
+    def apply_discount(self) -> None:
+        self.has_discount = True
+
+    @property
+    def value(self) -> Decimal:
+        if self.has_discount:
+            return self.unit_price * self.quantity * Decimal("0.8")
+        return self.unit_price * self.quantity
+
+
+class Order:
+    def __init__(self):
+        self.lines: list[OrderLine] = []
+        self.is_completed = False
+
+    @property
+    def total(self) -> Decimal:
+        return sum(line.value for line in self.lines)
+
+    def add_line(self, unit_price: Decimal, quantity: int) -> None:
+        self.lines.append(OrderLine(unit_price, quantity))
+
+    def apply_discount(self, line_index: int) -> None:
+        self.lines[line_index].apply_discount()
+
+    def complete(self) -> None:
+        self.is_completed = True
+
+    def is_fully_paid(self, expected_total: Decimal) -> bool:
+        return self.is_completed and self.total == expected_total

python/tests/order_tests/order_state.py

@@ -0,0 +1,32 @@
+"""Custom matcher that bundles multiple order assertions into one.
+
+Participants: you do not need to understand how this works —
+just recognize the pattern and use it as an example for the agent.
+"""
+
+from decimal import Decimal
+
+from order_domain import Order
+
+
+def assert_order_state(
+    order: Order, *, total: Decimal, line_values: list[Decimal]
+) -> None:
+    actual_total = order.total
+    actual_line_values = [line.value for line in order.lines]
+
+    details: list[str] = []
+    if actual_total != total:
+        details.append(f"Total: expected {total}, but was {actual_total}")
+    if actual_line_values != line_values:
+        details.append(
+            f"Line values: expected [{', '.join(str(v) for v in line_values)}], "
+            f"but was [{', '.join(str(v) for v in actual_line_values)}]"
+        )
+
+    if details:
+        header = (
+            f"Expected order with total {total} and line values "
+            f"[{', '.join(str(v) for v in line_values)}] but was:"
+        )
+        raise AssertionError("\n".join([header, *details]))

python/tests/order_tests/test_order_completion.py

@@ -0,0 +1,11 @@
+from decimal import Decimal
+
+from order_domain import Order
+
+
+def test_completed_order_total_is_correct():
+    order = Order()
+    order.add_line(Decimal("100"), 1)
+    order.complete()
+
+    assert order.is_fully_paid(Decimal("100"))

python/tests/order_tests/test_order_total.py

@@ -0,0 +1,77 @@
+from decimal import Decimal
+
+from order_domain import Order
+from order_state import assert_order_state
+
+
+# This test was refactored - notice the custom matcher.
+# The other tests still need the same treatment.
+def test_discount_on_first_line_total_and_line_values():
+    order = Order()
+    order.add_line(Decimal("100"), 2)  # line 0: 200 -> 160 with discount
+    order.add_line(Decimal("50"), 1)  # line 1: 50
+
+    order.apply_discount(0)
+
+    assert_order_state(
+        order, total=Decimal("210"), line_values=[Decimal("160"), Decimal("50")]
+    )
+
+
+def test_discount_on_second_line_total_and_line_values():
+    order = Order()
+    order.add_line(Decimal("100"), 2)  # line 0: 200
+    order.add_line(Decimal("50"), 1)  # line 1: 50 -> 40 with discount
+
+    order.apply_discount(1)
+
+    assert order.total == Decimal("240")
+    assert order.lines[0].value == Decimal("200")
+    assert order.lines[1].value == Decimal("40")
+
+
+def test_discount_on_both_lines_total_and_line_values():
+    order = Order()
+    order.add_line(Decimal("100"), 2)  # line 0: 200 -> 160
+    order.add_line(Decimal("50"), 1)  # line 1: 50 -> 40
+
+    order.apply_discount(0)
+    order.apply_discount(1)
+
+    assert order.total == Decimal("200")
+    assert order.lines[0].value == Decimal("160")
+    assert order.lines[1].value == Decimal("40")
+
+
+def test_no_discount_total_and_line_values():
+    order = Order()
+    order.add_line(Decimal("100"), 2)  # line 0: 200
+    order.add_line(Decimal("50"), 1)  # line 1: 50
+
+    assert order.total == Decimal("250")
+    assert order.lines[0].value == Decimal("200")
+    assert order.lines[1].value == Decimal("50")
+
+
+def test_single_line_with_discount_total_and_value():
+    order = Order()
+    order.add_line(Decimal("75"), 4)  # line 0: 300 -> 240
+
+    order.apply_discount(0)
+
+    assert order.total == Decimal("240")
+    assert order.lines[0].value == Decimal("240")
+
+
+def test_three_lines_one_discount_total_and_line_values():
+    order = Order()
+    order.add_line(Decimal("20"), 1)  # line 0: 20
+    order.add_line(Decimal("30"), 2)  # line 1: 60 -> 48
+    order.add_line(Decimal("10"), 3)  # line 2: 30
+
+    order.apply_discount(1)
+
+    assert order.total == Decimal("98")
+    assert order.lines[0].value == Decimal("20")
+    assert order.lines[1].value == Decimal("48")
+    assert order.lines[2].value == Decimal("30")