Move lazy_property to util package.

beets/library.py

@@ -24,14 +24,13 @@ import time
 import re
 import six
 import string
-import functools
 
 from beets import logging
 from beets.mediafile import MediaFile, UnreadableFileError
 from beets import plugins
 from beets import util
 from beets.util import bytestring_path, syspath, normpath, samefile, \
-    MoveOperation
+    MoveOperation, lazy_property
 from beets.util.functemplate import Template
 from beets import dbcore
 from beets.dbcore import types
@@ -369,22 +368,6 @@ 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.
 

beets/util/__init__.py

@@ -23,6 +23,7 @@ import locale
 import re
 import shutil
 import fnmatch
+import functools
 from collections import Counter
 from multiprocessing.pool import ThreadPool
 import traceback
@@ -1031,3 +1032,26 @@ def par_map(transform, items):
         pool.map(transform, items)
         pool.close()
         pool.join()
+
+
+def lazy_property(func):
+    """A decorator that creates a lazily evaluated property. On first access,
+    the property is assigned the return value of `func`. This first value is
+    stored, so that future accesses do not have to evaluate `func` again.
+
+    This behaviour is useful when `func` is expensive to evaluate, and it is
+    not certain that the result will be needed.
+    """
+    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