Exercise 3 solution

    import math
    import os
    import struct
    import tempfile
    import wave


    def write_tone(path, freq, secs, rate=8000, amp=0.5):
        n = int(rate * secs)
        pcm = b"".join(
            struct.pack("<h", round(amp * 32767 * math.sin(2 * math.pi * freq * i / rate)))
            for i in range(n)
        )
        with wave.open(path, "wb") as w:
            w.setnchannels(1)
            w.setsampwidth(2)
            w.setframerate(rate)
            w.writeframes(pcm)


    def read_by_hand(path):
        with open(path, "rb") as f:
            raw = f.read()
        if raw[:4] != b"RIFF" or raw[8:12] != b"WAVE":
            raise ValueError("not a WAV file")
        pos, chunks = 12, {}
        while pos + 8 <= len(raw):
            cid, size = struct.unpack_from("<4sI", raw, pos)
            chunks[cid] = raw[pos + 8:pos + 8 + size]
            pos += 8 + size + (size & 1)
        rate = struct.unpack_from("<HHI", chunks[b"fmt "])[2]
        data = chunks[b"data"]
        return rate, list(struct.unpack(f"<{len(data) // 2}h", data))


    folder = tempfile.mkdtemp()
    path = os.path.join(folder, "tone.wav")
    write_tone(path, 440, 0.25)

    with wave.open(path, "rb") as r:
        frames = r.getnframes()
        via_module = list(struct.unpack(f"<{frames}h", r.readframes(frames)))
        print(r.getframerate(), frames)

    rate, via_hand = read_by_hand(path)
    print(rate, len(via_hand))
    print(via_hand == via_module)
    print(max(via_hand), min(via_hand))
    print(os.path.getsize(path) == 44 + 2 * frames)

    os.remove(path)
    os.rmdir(folder)
    print(os.path.exists(folder))

    # 8000 2000
    # 8000 2000
    # True
    # 16384 -16384
    # True
    # False

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

Explanation: The tone is a sine wave scaled to half of the 16-bit range (about 16384),
packed as little-endian signed 16-bit samples and handed to the wave module, which
writes the RIFF, fmt and data chunks. The hand-written reader walks those same chunks
and unpacks the samples itself, and the two readings agree exactly. The file size is
the 44-byte header plus 2 bytes per frame. The folder is removed afterwards.