Challenge 3: Fix the Mutable Default Bug — Possible Solution
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def add_item(item, basket=None):
    if basket is None:
        basket = []
    basket.append(item)
    return basket

print(add_item("apple"))    # ['apple']
print(add_item("banana"))   # ['banana'] — a fresh, independent list this time

WHY THIS WORKS AS AN ANSWER
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The buggy version's problem, per the chapter's warn-box, is that
basket=[] is evaluated exactly once when the function is defined, so
every call that relies on the default ends up sharing and mutating
that same one list across calls.

The fix replaces the mutable default with an immutable one instead:
basket=None. None is a single, immutable value that's perfectly safe
to reuse as a default across every call, unlike a list.

if basket is None: basket = [] then runs at the START of the function
body, on every single call, not just once at definition time. So each
call that doesn't supply its own basket gets a genuinely brand-new
empty list created fresh right then — which is why add_item("banana")
correctly returns ['banana'] instead of ['apple', 'banana'] the way
the buggy version did.

If a caller DOES pass their own basket explicitly, basket is None is
False, so that provided list is used as-is and the fresh-list branch
is skipped entirely — the fix only changes behavior for the
default-omitted case, exactly as intended.