Exercise 2: Adding save_refund() Behind the Existing Boundary — Possible Solution
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THE NEW REPOSITORY METHODS
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    # FileOrderRepository
    def save_refund(self, order_id, amount):
        with open('refunds.txt', 'a') as f:
            f.write(f'{order_id},{amount}\n')

    # InMemoryOrderRepository
    def save_refund(self, order_id, amount):
        self.refunds.append(f'{order_id},{amount}')

Each repository gets its own save_refund(), storage-specific exactly
like this chapter's own save_order()/save_payment() pair - the file
version writes to refunds.txt, the in-memory version appends to a new
self.refunds list.

THE NEW BUSINESS FUNCTION
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    def record_refund(repo, order_id, amount):
        repo.save_refund(order_id, amount)

Matches this chapter's own record_order()/record_payment() shape
exactly - it takes an injected repo and delegates, mentioning no
storage technology at all.

VERIFYING NO open() CALL IN THE NEW BUSINESS FUNCTION
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    record_refund contains open(): False

Confirmed directly via inspect.getsource(), exactly like this
chapter's own verification of the three original business functions.

VERIFYING IT WORKS AGAINST BOTH REPOSITORIES
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Calling record_refund(file_repo, 'ORD-1', 20) and
record_refund(memory_repo, 'ORD-1', 20) with the identical business
function:

    memory_repo.refunds: ['ORD-1,20']
    refunds.txt contents: ['ORD-1,20']

Both repositories correctly recorded the refund using their own
storage mechanism, and record_refund() itself never needed to know
which one it was talking to.

WHY THIS WORKS AS AN ANSWER
------------------------------
The new capability is added entirely within the existing boundary -
one new method per repository, one new business function that only
delegates - following this chapter's own established shape exactly,
and both the "no open() call" claim and the "works against both
repositories" claim are verified directly rather than assumed to
follow from the pattern being followed correctly.