feat: YouTube-Untertitel herunterladen (yt_untertitel.py)

- yt_untertitel.py lädt über yt-dlp die Untertitel eines Videos als VTT:
  manuelle Untertitel in der Sprache, sonst automatische in Originalsprache
  (<lang>-orig); Maschinenübersetzungen werden nie genommen
- Tests ohne Netz (yt-dlp durch Fakes ersetzt)
- Testdaten: Lahnstein-Rede (VTT und daraus erzeugter Rohtext)
- README: Ablauf URL → VTT → TXT → Korrektur

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_yt_untertitel.py

@@ -0,0 +1,138 @@
+"""Tests for yt_untertitel.py; yt-dlp is always replaced by fakes (no network)."""
+import json
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+import yt_untertitel as yt
+
+URL = "https://www.youtube.com/watch?v=abc123"
+
+AUTO_ONLY = {
+    "id": "abc123",
+    "title": "Eine Rede",
+    "subtitles": {},
+    "automatic_captions": {"de-orig": [], "de": [], "en": []},
+}
+
+
+def fake_yt_dlp(info, write=True, fail_download=False):
+    """Stand-in for subprocess.run: answers the info request, writes the VTT file."""
+    calls = []
+
+    def run(command, capture_output, text):
+        calls.append(command)
+        if "--dump-single-json" in command:
+            return subprocess.CompletedProcess(command, 0, json.dumps(info), "")
+        if fail_download:
+            return subprocess.CompletedProcess(command, 1, "", "HTTP Error 429")
+        track = command[command.index("--sub-langs") + 1]
+        template = command[command.index("-o") + 1]
+        if write:
+            Path(template.replace("%(ext)s", f"{track}.vtt")).write_text(
+                "WEBVTT\n", encoding="utf-8"
+            )
+        return subprocess.CompletedProcess(command, 0, "", "")
+
+    run.calls = calls
+    return run
+
+
+@pytest.fixture(autouse=True)
+def yt_dlp_installed(monkeypatch):
+    monkeypatch.setattr(yt.shutil, "which", lambda name: f"/usr/bin/{name}")
+
+
+def run_main(monkeypatch, *args):
+    monkeypatch.setattr(sys, "argv", ["yt_untertitel.py", *map(str, args)])
+    yt.main()
+
+
+# === Track selection ===
+
+@pytest.mark.parametrize("info, lang, expected", [
+    ({"subtitles": {"de": []}, "automatic_captions": {"de-orig": []}}, "de", ("de", False)),
+    (AUTO_ONLY, "de", ("de-orig", True)),
+    # "en" exists only as machine translation of the German original
+    (AUTO_ONLY, "en", None),
+    ({"subtitles": None, "automatic_captions": None}, "de", None),
+])
+def test_choose_track(info, lang, expected):
+    assert yt.choose_track(info, lang) == expected
+
+
+def test_available_tracks():
+    info = {"subtitles": {"fr": []}, "automatic_captions": {"de-orig": [], "de": []}}
+    assert yt.available_tracks(info) == (["fr"], ["de"])
+
+
+# === Command line ===
+
+def test_main_downloads_auto_original(tmp_path, monkeypatch, capsys):
+    run = fake_yt_dlp(AUTO_ONLY)
+    monkeypatch.setattr(yt.subprocess, "run", run)
+    run_main(monkeypatch, "-u", URL, "-n", "Rede", "-d", tmp_path / "neu")
+
+    assert (tmp_path / "neu" / "Rede.de-orig.vtt").is_file()
+    download = run.calls[1]
+    assert "--write-auto-subs" in download and "--write-subs" not in download
+    assert download[-1] == URL
+    out = capsys.readouterr().out
+    assert "automatische Untertitel „de-orig“" in out
+    assert "vtt2txt.py -i" in out
+
+
+def test_main_prefers_manual_and_defaults_to_video_id(tmp_path, monkeypatch):
+    info = dict(AUTO_ONLY, subtitles={"de": []})
+    run = fake_yt_dlp(info)
+    monkeypatch.setattr(yt.subprocess, "run", run)
+    run_main(monkeypatch, "-u", URL, "-d", tmp_path)
+
+    assert (tmp_path / "abc123.de.vtt").is_file()
+    assert "--write-subs" in run.calls[1]
+
+
+def test_main_no_matching_track(tmp_path, monkeypatch):
+    monkeypatch.setattr(yt.subprocess, "run", fake_yt_dlp(AUTO_ONLY))
+    with pytest.raises(SystemExit, match="Automatisch \\(Originalsprache\\): de"):
+        run_main(monkeypatch, "-u", URL, "-l", "en", "-d", tmp_path)
+
+
+def test_main_info_error(tmp_path, monkeypatch):
+    def run(command, capture_output, text):
+        return subprocess.CompletedProcess(command, 1, "", "ERROR: Video unavailable")
+
+    monkeypatch.setattr(yt.subprocess, "run", run)
+    with pytest.raises(SystemExit, match="Video unavailable"):
+        run_main(monkeypatch, "-u", URL, "-d", tmp_path)
+
+
+@pytest.mark.parametrize("kwargs, message", [
+    ({"fail_download": True}, "HTTP Error 429"),
+    ({"write": False}, "keine Datei"),
+])
+def test_main_download_errors(tmp_path, monkeypatch, kwargs, message):
+    monkeypatch.setattr(yt.subprocess, "run", fake_yt_dlp(AUTO_ONLY, **kwargs))
+    with pytest.raises(SystemExit, match=message):
+        run_main(monkeypatch, "-u", URL, "-d", tmp_path)
+
+
+@pytest.mark.parametrize("args, message", [
+    ([], "required"),
+    (["-u", URL, "-n", "sub/Rede"], "ohne Verzeichnis"),
+    (["-u", URL, "-n", ""], "ohne Verzeichnis"),
+])
+def test_cli_errors(monkeypatch, capsys, args, message):
+    with pytest.raises(SystemExit) as exc:
+        run_main(monkeypatch, *args)
+    assert exc.value.code == 2
+    assert message in capsys.readouterr().err
+
+
+def test_cli_without_yt_dlp(monkeypatch, capsys):
+    monkeypatch.setattr(yt.shutil, "which", lambda name: None)
+    with pytest.raises(SystemExit):
+        run_main(monkeypatch, "-u", URL)
+    assert "pip install yt-dlp" in capsys.readouterr().err