wave: Reject parameter changes after empty frame writes

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The wave documentation, under Wave_write objects, states: "Note that it is invalid to set any parameters after calling writeframes() or writeframesraw(), and any attempt to do so will raise wave.Error."

Expected: setnchannels(2) raises wave.Error after a successful writeframes(b'') or writeframesraw(b'').

Actual: After writeframes(b''), setnchannels(2) succeeds without an exception.

import io, wave

p = dict(channels=1, sample_width=1, frame_rate=8000,
         method="writeframes", data=b"", new_channels=2)
try:
    if not (p["channels"] > 0 and 1 <= p["sample_width"] <= 4
            and p["frame_rate"] > 0 and p["new_channels"] > 0
            and p["new_channels"] != p["channels"]
            and isinstance(p["data"], bytes)):
        raise ValueError("invalid PCM input")
    w = wave.open(io.BytesIO(), "wb")
    w.setparams((p["channels"], p["sample_width"], p["frame_rate"],
                 0, "NONE", "not compressed"))
    getattr(w, p["method"])(p["data"])
    expected = "raises wave.Error"  # Documentation forbids changes after either write.
    try:
        w.setnchannels(p["new_channels"])
        actual = "no exception"
    except wave.Error:
        actual = "raises wave.Error"
    if actual != expected:
        print("REFUTATION CONFIRMED:", p, "actual:", actual, "expected:", expected)
    else:
        print("REFUTATION REJECTED: documented wave.Error was raised")
except Exception as e:
    print("REFUTATION REJECTED:", type(e).__name__, str(e))

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library wave:

REFUTATION CONFIRMED: {'channels': 1, 'sample_width': 1, 'frame_rate': 8000, 'method': 'writeframes', 'data': b'', 'new_channels': 2} actual: no exception expected: raises wave.Error

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/wave-20261003-042850-c2

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).