Exercise 2: A Conditional NewCustomerDiscount Strategy — Possible Solution
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THE NEW STRATEGY
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    class NewCustomerDiscount:
        def apply(self, total):
            return 5 if total > 50 else 0

Follows the exact same one-method apply(total) interface as
VipDiscount, LoyaltyDiscount, and NoDiscount - the only difference is
that its own logic is conditional on its input, something none of the
three existing strategies needed to do. Nothing about Order or
OrderBuilder had to change to accommodate this - both already just
call self.discount_strategy.apply(self.items_total) without caring
what happens inside.

VERIFYING THE OVER-$50 CASE
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    order_over = OrderBuilder().items_total(80).weight(4).discount_strategy(NewCustomerDiscount()).build()

    over-50 total: 77.0
    manual: 80 - 5 + 4*0.5 = 77.0

VERIFYING THE UNDER-$50 CASE
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    order_under = OrderBuilder().items_total(30).weight(4).discount_strategy(NewCustomerDiscount()).build()

    under-50 total: 32.0
    manual: 30 - 0 + 4*0.5 = 32.0

Both cases match their hand calculations exactly - the $5 discount
correctly applies only when items_total exceeds $50, and correctly
does not apply otherwise.

WHY THIS CONFIRMS THIS CHAPTER'S OWN CLAIM
------------------------------
This exercise adds a genuinely new business rule (a threshold-based
discount, unlike any of the three existing flat-rate/percentage
strategies) without touching Order, OrderBuilder, or any of the three
existing discount classes at all - directly demonstrating this
chapter's own "Where This Wasn't Used" reasoning that Strategy was the
right tool: new discount rules are added by writing one new,
self-contained class, not by editing shared code.

WHY THIS WORKS AS AN ANSWER
------------------------------
The new strategy matches the established apply(total) interface
exactly while adding genuinely new conditional logic, and both branches
of that condition are verified separately against hand calculations
rather than only checking one case.