Move LRUCache to picard/utils/lrucache.py

Moved the LRUCache class to its own module in utils, in case it's useful
in other parts of the code.

picard/ui/coverartbox.py

@@ -26,7 +26,8 @@ from picard.album import Album
 from picard.coverart.image import CoverArtImage, CoverArtImageError
 from picard.track import Track
 from picard.file import File
-from picard.util import encode_filename, imageinfo
+from picard.util import encode_filename, imageinfo, get_file_path
+from picard.util.lrucache import LRUCache
 
 if sys.platform == 'darwin':
     try:
@@ -37,38 +38,7 @@ if sys.platform == 'darwin':
         log.warning("Unable to import NSURL, file drag'n'drop might not work correctly")
 
 
-class LRUCache(dict):
-
-    def __init__(self, max_size):
-        self._ordered_keys = []
-        self._max_size = max_size
-
-    def __getitem__(self, key):
-        if key in self:
-            self._ordered_keys.remove(key)
-            self._ordered_keys.insert(0, key)
-        return super(LRUCache, self).__getitem__(key)
-
-    def __setitem__(self, key, value):
-        if key in self:
-            self._ordered_keys.remove(key)
-        self._ordered_keys.insert(0, key)
-
-        r = super(LRUCache, self).__setitem__(key, value)
-
-        if len(self) > self._max_size:
-            item = self._ordered_keys.pop()
-            super(LRUCache, self).__delitem__(item)
-
-        return r
-
-    def __delitem__(self, key):
-        self._ordered_keys.remove(key)
-        super(LRUCache, self).__delitem__(key)
-
-
 class ActiveLabel(QtGui.QLabel):
-
     """Clickable QLabel."""
 
     clicked = QtCore.pyqtSignal()

picard/util/lrucache.py

@@ -0,0 +1,79 @@
+# -*- coding: utf-8 -*-
+#
+# Picard, the next-generation MusicBrainz tagger
+# Copyright (C) 2017 Antonio Larrosa <alarrosa@suse.com>
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+
+"""
+Helper class to cache items using a Least Recently Used policy.
+
+It's originally used to cache generated pixmaps in the CoverArtBox object
+but it's generic enough to be used for other purposes if necessary.
+The cache will never hold more than max_size items and the item least
+recently used will be discarded.
+
+>>> cache = LRUCache(3)
+>>> cache['item1'] = 'some value'
+>>> cache['item2'] = 'some other value'
+>>> cache['item3'] = 'yet another value'
+>>> cache['item1']
+'some value'
+>>> cache['item4'] = 'This will push item 2 out of the cache'
+>>> cache['item2']
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "lrucache.py", line 48, in __getitem__
+    return super(LRUCache, self).__getitem__(key)
+KeyError: 'item2'
+>>> cache['item5'] = 'This will push item3 out of the cache'
+>>> cache['item3']
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "lrucache.py", line 48, in __getitem__
+    return super(LRUCache, self).__getitem__(key)
+KeyError: 'item3'
+>>> cache['item1']
+'some value'
+"""
+
+
+class LRUCache(dict):
+    def __init__(self, max_size):
+        self._ordered_keys = []
+        self._max_size = max_size
+
+    def __getitem__(self, key):
+        if key in self:
+            self._ordered_keys.remove(key)
+            self._ordered_keys.insert(0, key)
+        return super(LRUCache, self).__getitem__(key)
+
+    def __setitem__(self, key, value):
+        if key in self:
+            self._ordered_keys.remove(key)
+        self._ordered_keys.insert(0, key)
+
+        r = super(LRUCache, self).__setitem__(key, value)
+
+        if len(self) > self._max_size:
+            item = self._ordered_keys.pop()
+            super(LRUCache, self).__delitem__(item)
+
+        return r
+
+    def __delitem__(self, key):
+        self._ordered_keys.remove(key)
+        super(LRUCache, self).__delitem__(key)