Exercise 2: A Third Function Reproducing the Same Bug Class — Possible Solution
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THE NEW FUNCTION
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    def format_receipt_line(price_dollars):
        return f"Total due: ${price_dollars:.2f}"

Composed directly with the chapter's own calculate_price_in_cents
(which returns cents, not dollars):

    item = {'price_dollars': 19.99}
    cents = calculate_price_in_cents(item)          # 1999
    buggy_line = format_receipt_line(cents)          # treats 1999 as dollars

RESULTS
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    BUGGY composition: "Total due: $1999.00"
    (should read approximately $19.99 - the same class of unit
    mismatch as the chapter's own apply_discount_dollars example)

    FIXED composition (converting cents back to dollars first):
        def checkout_total_dollars(item):
            cents = calculate_price_in_cents(item)
            return cents / 100

        fixed_line = format_receipt_line(checkout_total_dollars(item))

    FIXED composition: "Total due: $19.99"

WHY THIS WORKS AS AN ANSWER
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This confirms the chapter's own finding isn't specific to
apply_discount_dollars - any function downstream of
calculate_price_in_cents that expects dollars reproduces the identical
bug class, because the actual defect lives at the interface (an
undocumented unit mismatch), not inside either individual function.
Both format_receipt_line and apply_discount_dollars are, individually,
completely correct - the bug only exists in how they're wired
together, exactly the chapter's own point about composition versus
unit-level correctness.