Cache 'no artwork', rather than repeatedly populating the cache with None

src/mpd_now_playable/mpd/artwork_cache.py

@@ -1,3 +1,5 @@
+from dataclasses import dataclass
+
 from aiocache import Cache
 
 from ..async_tools import run_background_task
@@ -6,6 +8,25 @@ from .types import CurrentSongResponse, MpdStateHandler
 CACHE_TTL = 60 * 10  # ten minutes
 
 
+@dataclass(frozen=True)
+class HasArt:
+	data: bytes
+
+
+@dataclass(frozen=True)
+class HasNoArt:
+	data = None
+
+
+ArtCacheEntry = HasArt | HasNoArt
+
+
+def make_cache_entry(art: bytes | None) -> ArtCacheEntry:
+	if art is None:
+		return HasNoArt()
+	return HasArt(art)
+
+
 def calc_album_key(song: CurrentSongResponse) -> str:
 	artist = song.get("albumartist", song.get("artist", "Unknown Artist"))
 	album = song.get("album", "Unknown Album")
@@ -18,8 +39,8 @@ def calc_track_key(song: CurrentSongResponse) -> str:
 
 class MpdArtworkCache:
 	mpd: MpdStateHandler
-	album_cache: "Cache[bytes | None]"
-	track_cache: "Cache[bytes | None]"
+	album_cache: "Cache[ArtCacheEntry]"
+	track_cache: "Cache[ArtCacheEntry]"
 
 	def __init__(self, mpd: MpdStateHandler):
 		self.mpd = mpd
@@ -29,7 +50,7 @@ class MpdArtworkCache:
 	async def get_cached_artwork(self, song: CurrentSongResponse) -> bytes | None:
 		art = await self.track_cache.get(calc_track_key(song))
 		if art:
-			return art
+			return art.data
 
 		# If we don't have track artwork cached, go find some.
 		run_background_task(self.cache_artwork(song))
@@ -37,12 +58,12 @@ class MpdArtworkCache:
 		# Even if we don't have cached track art, we can try looking for cached album art.
 		art = await self.album_cache.get(calc_album_key(song))
 		if art:
-			return art
+			return art.data
 
 		return None
 
 	async def cache_artwork(self, song: CurrentSongResponse) -> None:
-		art = await self.mpd.readpicture(song["file"])
+		art = make_cache_entry(await self.mpd.readpicture(song["file"]))
 		try:
 			await self.album_cache.add(calc_album_key(song), art, ttl=CACHE_TTL)
 		except ValueError: