Introduce @lazy_property for better readability.

beets/library.py

@@ -24,6 +24,7 @@ import time
 import re
 import six
 import string
+import functools
 
 from beets import logging
 from beets.mediafile import MediaFile, UnreadableFileError
@@ -368,6 +369,22 @@ class LibModel(dbcore.Model):
         return self.__str__().encode('utf-8')
 
 
+def lazy_property(func):
+    field_name = '_' + func.__name__
+
+    @property
+    @functools.wraps(func)
+    def wrapper(self):
+        if hasattr(self, field_name):
+            return getattr(self, field_name)
+
+        value = func(self)
+        setattr(self, field_name, value)
+        return value
+
+    return wrapper
+
+
 class FormattedItemMapping(dbcore.db.FormattedMapping):
     """Add lookup for album-level fields.
 
@@ -376,42 +393,36 @@ class FormattedItemMapping(dbcore.db.FormattedMapping):
 
     def __init__(self, item, for_path=False):
         super(FormattedItemMapping, self).__init__(item, for_path)
-        self.album = None
-        self.album_keys = None
-        self.all_keys = None
         self.item = item
 
-    def _all_keys(self):
-        if not self.all_keys:
-            self.all_keys = set(self.model_keys).union(self._album_keys())
-        return self.all_keys
+    @lazy_property
+    def all_keys(self):
+        return set(self.model_keys).union(self.album_keys)
 
-    def _album_keys(self):
-        if not self.album_keys:
-            album = self._album()
-            self.album_keys = []
-            if album:
-                for key in album.keys(True):
-                    if key in Album.item_keys \
-                            or key not in self.item._fields.keys():
-                        self.album_keys.append(key)
-        return self.album_keys
+    @lazy_property
+    def album_keys(self):
+        album_keys = []
+        if self.album:
+            for key in self.album.keys(True):
+                if key in Album.item_keys \
+                    or key not in self.item._fields.keys():
+                    album_keys.append(key)
+        return album_keys
 
-    def _album(self):
-        if not self.album:
-            self.album = self.item.get_album()
-        return self.album
+    @lazy_property
+    def album(self):
+        return self.item.get_album()
 
     def _get(self, key):
         """Get the value for a key, either from the album or the item.
         Raise a KeyError for invalid keys.
         """
-        if self.for_path and key in self._album_keys():
-            return self._get_formatted(self._album(), key)
+        if self.for_path and key in self.album_keys:
+            return self._get_formatted(self.album, key)
         elif key in self.model_keys:
             return self._get_formatted(self.model, key)
-        elif key in self._album_keys():
-            return self._get_formatted(self._album(), key)
+        elif key in self.album_keys:
+            return self._get_formatted(self.album, key)
         else:
             raise KeyError(key)
 
@@ -431,10 +442,10 @@ class FormattedItemMapping(dbcore.db.FormattedMapping):
             return value
 
     def __iter__(self):
-        return iter(self._all_keys())
+        return iter(self.all_keys)
 
     def __len__(self):
-        return len(self._all_keys())
+        return len(self.all_keys)
 
 
 class Item(LibModel):