mpdrandom supports prefix

this is useful if your music library is bigger than what you want to
play.

larigira/audiogen_mpdrandom.py

@@ -1,25 +1,40 @@
 import logging
-
-log = logging.getLogger("mpdrandom")
 import random
 
 from mpd import MPDClient
 
 from .config import get_conf
 
+log = logging.getLogger(__name__)
+
 
 def generate_by_artist(spec):
     """choose HOWMANY random artists, and for each one choose a random song"""
+
     spec.setdefault("howmany", 1)
+    prefix = spec.get("prefix", "").rstrip("/")
     log.info("generating")
     conf = get_conf()
     c = MPDClient(use_unicode=True)
     c.connect(conf["MPD_HOST"], conf["MPD_PORT"])
 
-    artists = c.list("artist")
-    log.debug("got %d artists", len(artists))
+    if prefix:
+        # TODO: listallinfo is discouraged. how else could we achieve the same result?
+        artists = list(
+            {r["artist"] for r in c.listallinfo(prefix) if "artist" in r}
+        )
+    else:
+        artists = c.list("artist")
     if not artists:
         raise ValueError("no artists in your mpd database")
     for _ in range(spec["howmany"]):
-        artist = random.choice(artists)
-        yield random.choice(c.find("artist", artist))["file"]
+        artist = random.choice(artists)  # pick one artist
+        # pick one song from that artist
+        artist_songs = (res["file"] for res in c.find("artist", artist))
+        if prefix:
+            artist_songs = [
+                fname
+                for fname in artist_songs
+                if fname.startswith(prefix + "/")
+            ]
+        yield random.choice(list(artist_songs))