fix: route sparse-metadata playlist tracks by yt-dlp's own metadata

SoundCloud sets (and similar) return flat-playlist entries without per-track
artist/title. When a track Hit has no artist, download via an output template
(-o <root>/%(artist,uploader,channel)s/<source>/...) so yt-dlp places the file
under the real artist instead of "Unknown Artist". yt_download gains an optional
outtmpl mode.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

musicfetch

@@ -593,14 +593,18 @@ def _quality_args(quality: str) -> list[str]:
     return ["-f", "bestaudio/best", "-x"]
 
 
-def yt_download(url_or_query: str, target_folder: str, quality: str, dry_run: bool,
-                hit: Optional[Hit] = None):
+def yt_download(url_or_query: str, target_folder: Optional[str], quality: str, dry_run: bool,
+                hit: Optional[Hit] = None, outtmpl: Optional[str] = None):
     cmd = ["yt-dlp",
            *_quality_args(quality),
            "--embed-metadata",
            "--embed-thumbnail",
-           "--no-playlist",
-           "-P", target_folder]
+           "--no-playlist"]
+    # Either a fixed output dir (-P) or a metadata-driven output template (-o).
+    if outtmpl:
+        cmd += ["-o", outtmpl]
+    else:
+        cmd += ["-P", target_folder]
     # Override tags from the chosen hit so they don't rely on scraped titles.
     if hit:
         if hit.artist:
@@ -616,12 +620,15 @@ def yt_download(url_or_query: str, target_folder: str, quality: str, dry_run: bo
             cmd += ["--parse-metadata", f"{hit.year}:%(release_year)s"]
     cmd.append(url_or_query)
 
+    dest = outtmpl or target_folder
     if dry_run:
-        print(f"[dry-run] mkdir -p {target_folder}")
+        if target_folder:
+            print(f"[dry-run] mkdir -p {target_folder}")
         print(f"[dry-run] {' '.join(cmd)}")
         return True
-    os.makedirs(target_folder, exist_ok=True)
-    print(f"Downloading via yt-dlp -> {target_folder}")
+    if target_folder:
+        os.makedirs(target_folder, exist_ok=True)
+    print(f"Downloading via yt-dlp -> {dest}")
     return subprocess.run(cmd).returncode == 0
 
 
@@ -648,10 +655,15 @@ def _track_url(hit: Hit) -> str:
 
 def act_youtube(hit: Hit, root: str, quality: str, dry_run: bool):
     url = _track_url(hit)
-    artist_dir = hit.artist.split(",")[0].strip() or "Unknown Artist"
     source = hit.payload.get("extractor") or "youtube"
-    target = os.path.join(root, artist_dir, source)
-    return yt_download(url, target, quality, dry_run, hit=hit)
+    artist_dir = hit.artist.split(",")[0].strip()
+    if artist_dir:
+        target = os.path.join(root, artist_dir, source)
+        return yt_download(url, target, quality, dry_run, hit=hit)
+    # Sparse playlist metadata (e.g. SoundCloud sets): let yt-dlp route the file
+    # by the track's own metadata so it lands under the real artist.
+    outtmpl = os.path.join(root, "%(artist,uploader,channel)s", source, "%(title)s [%(id)s].%(ext)s")
+    return yt_download(url, None, quality, dry_run, hit=hit, outtmpl=outtmpl)
 
 
 # ---------------------------------------------------------------------------

tests/test_multiplatform.py

@@ -60,13 +60,27 @@ def test_act_youtube_soundcloud_folder(monkeypatch):
 def test_act_youtube_youtube_folder(monkeypatch):
     captured = {}
     monkeypatch.setattr(mf, "yt_download",
-                        lambda url, target, quality, dry_run, hit=None: captured.update(target=target) or True)
+                        lambda url, target, quality, dry_run, hit=None, outtmpl=None:
+                        captured.update(target=target) or True)
     h = mf.Hit(source="youtube", kind="track", title="T", artist="A",
                payload={"videoId": "vid", "extractor": "youtube"})
     mf.act_youtube(h, "/media/music", "best", False)
     assert captured["target"] == "/media/music/A/youtube"
 
 
+def test_act_youtube_unknown_artist_uses_metadata_template(monkeypatch):
+    captured = {}
+    monkeypatch.setattr(mf, "yt_download",
+                        lambda url, target, quality, dry_run, hit=None, outtmpl=None:
+                        captured.update(target=target, outtmpl=outtmpl) or True)
+    h = mf.Hit(source="youtube", kind="track", title="", artist="",
+               payload={"videoId": None, "extractor": "soundcloud", "url": "https://soundcloud.com/a/t"})
+    mf.act_youtube(h, "/media/music", "best", False)
+    assert captured["target"] is None
+    assert "%(artist,uploader,channel)s" in captured["outtmpl"]
+    assert captured["outtmpl"].endswith("/soundcloud/%(title)s [%(id)s].%(ext)s")
+
+
 # ---- download_single per-source folder ----
 def test_download_single_bandcamp_folder(monkeypatch):
     monkeypatch.setattr(mf, "run_yt_dlp_get_metadata",