Validate filename template placeholders in config

src/luna/config.py

@@ -1,9 +1,11 @@
 from dataclasses import dataclass, asdict
 from pathlib import Path
+from string import Formatter
 import json
 
 DEFAULT_EXTENSIONS = (".mp3", ".flac", ".m4a", ".aac", ".ogg", ".opus", ".wav", ".wma")
 _CONFIG_FIELDS = {"roots", "extensions", "ignored_paths", "max_workers", "filename_template"}
+_SUPPORTED_FILENAME_FIELDS = {"track", "artist", "title", "album", "album_artist", "disc", "year", "genre", "format"}
 
 
 @dataclass
@@ -19,6 +21,20 @@ def defaults(cls):
         return cls([], list(DEFAULT_EXTENSIONS), [])
 
 
+def _validate_filename_template(template: str) -> None:
+    formatter = Formatter()
+    try:
+        parts = list(formatter.parse(template))
+    except ValueError as exc:
+        raise ValueError(f"Configuration 'filename_template' is invalid: {exc}") from exc
+
+    for _, field_name, format_spec, _ in parts:
+        if field_name is not None and field_name not in _SUPPORTED_FILENAME_FIELDS:
+            raise ValueError(f"Configuration 'filename_template' uses unsupported field: {field_name}")
+        if format_spec:
+            _validate_filename_template(format_spec)
+
+
 def validate_profile(profile: LibraryProfile) -> LibraryProfile:
     if not isinstance(profile.roots, list) or not all(isinstance(value, str) and value.strip() for value in profile.roots):
         raise ValueError("Configuration 'roots' must be a list of non-empty strings.")
@@ -30,6 +46,7 @@ def validate_profile(profile: LibraryProfile) -> LibraryProfile:
         raise ValueError("Configuration 'max_workers' must be a positive integer.")
     if not isinstance(profile.filename_template, str) or not profile.filename_template.strip():
         raise ValueError("Configuration 'filename_template' must be a non-empty string.")
+    _validate_filename_template(profile.filename_template)
     return profile
 
 

tests/test_config.py

@@ -30,6 +30,22 @@ def test_invalid_filename_template_is_rejected(self):
         with self.assertRaisesRegex(ValueError, "filename_template.*non-empty string"):
             validate_profile(LibraryProfile([], [".mp3"], [], filename_template=""))
 
+    def test_all_supported_filename_placeholders_are_valid(self):
+        template = "{track} {artist} {title} {album} {album_artist} {disc} {year} {genre} {format}"
+        self.assertEqual(validate_profile(LibraryProfile([], [".mp3"], [], filename_template=template)).filename_template, template)
+
+    def test_unsupported_filename_placeholder_is_rejected(self):
+        with self.assertRaisesRegex(ValueError, "filename_template.*unsupported field.*banana"):
+            validate_profile(LibraryProfile([], [".mp3"], [], filename_template="{banana}"))
+
+    def test_malformed_filename_template_is_rejected(self):
+        with self.assertRaisesRegex(ValueError, "filename_template.*invalid"):
+            validate_profile(LibraryProfile([], [".mp3"], [], filename_template="{title"))
+
+    def test_nested_unsupported_filename_placeholder_is_rejected(self):
+        with self.assertRaisesRegex(ValueError, "filename_template.*unsupported field.*width"):
+            validate_profile(LibraryProfile([], [".mp3"], [], filename_template="{title:{width}}"))
+
     def test_unknown_configuration_fields_are_rejected(self):
         with tempfile.TemporaryDirectory() as tmp:
             path = Path(tmp) / "config.json"