Exercise 2 solution

    import os
    import struct
    import tempfile

    HEADER = struct.Struct("<4sBBHI")
    MAGIC = b"DEMO"


    def write_demo(path, major, minor, flags, records):
        with open(path, "wb") as f:
            f.write(HEADER.pack(MAGIC, major, minor, flags, len(records)))
            for r in records:
                f.write(struct.pack("<H", r))


    def read_demo(path):
        with open(path, "rb") as f:
            raw = f.read(HEADER.size)
            if len(raw) < HEADER.size:
                raise ValueError(f"truncated header: {len(raw)} of {HEADER.size} bytes")
            magic, major, minor, flags, count = HEADER.unpack(raw)
            if magic != MAGIC:
                raise ValueError(f"bad magic {magic!r}")
            if major != 1:
                raise ValueError(f"unsupported major version {major}")
            body = f.read(count * 2)
        if len(body) != count * 2:
            raise ValueError(f"expected {count} records, body holds {len(body) // 2}")
        values = list(struct.unpack(f"<{count}H", body))
        return (major, minor), flags, values


    path = os.path.join(tempfile.gettempdir(), "demo_ex2.bin")
    try:
        write_demo(path, 1, 0, 3, [10, 20, 30])
        print(read_demo(path))
        write_demo(path, 1, 7, 0, [])
        print(read_demo(path))

        write_demo(path, 2, 0, 0, [1])
        cases = {"major 2": None, "bad magic": b"XXXX" + bytes(8), "truncated header": b"DEMO\x01",
                 "short body": HEADER.pack(MAGIC, 1, 0, 0, 5) + b"\x01\x00"}
        for label, data in cases.items():
            if data is not None:
                with open(path, "wb") as f:
                    f.write(data)
            try:
                read_demo(path)
            except ValueError as e:
                print(label + ":", e)
    finally:
        os.remove(path)
    print(os.path.exists(path))

    # ((1, 0), 3, [10, 20, 30])
    # ((1, 7), 0, [])
    # major 2: unsupported major version 2
    # bad magic: bad magic b'XXXX'
    # truncated header: truncated header: 5 of 12 bytes
    # short body: expected 5 records, body holds 1
    # False

(Run as written; the output matched the comments.)

Explanation: read_demo checks things in the order the file can fail: first that
the 12 header bytes exist at all, then the magic, then the major version, and
only then does it trust the record count to read the body. A different minor
version (1.7) is accepted because a minor bump promises compatibility, while a
different major version is refused. The final check compares the body length with
the count the header claims, so a file cut short is reported instead of
unpacked wrongly.