Exercise 1 solution

    def can_be_key(value):
        try:
            hash(value)
            return True
        except TypeError:
            return False

    for v in [5, "hi", (1, 2), [1, 2], (1, [2]), {1}, frozenset({1}), None]:
        print(repr(v), can_be_key(v))

Output:
    5 True
    'hi' True
    (1, 2) True
    [1, 2] False
    (1, [2]) False
    {1} False
    frozenset({1}) True
    None True

The tuple rule explains (1, 2) -> True and (1, [2]) -> False: a tuple is
hashable only when everything inside it is hashable. The list inside the
second tuple makes the whole tuple unhashable.