Use a simpler version of the cached_property decorator.

From https://github.com/pydanny/cached-propertyi. This fixes an issue with
docstrings.

sigal/gallery.py

@@ -147,16 +147,16 @@ class Image(Media):
     type = 'image'
     extensions = ('.jpg', '.jpeg', '.png')
 
-    @cached_property()
+    @cached_property
     def date(self):
         return self.exif and self.exif.get('dateobj', None) or None
 
-    @cached_property()
+    @cached_property
     def exif(self):
         return (get_exif_tags(self.raw_exif)
                 if self.raw_exif and self.ext in ('.jpg', '.jpeg') else None)
 
-    @cached_property()
+    @cached_property
     def raw_exif(self):
         try:
             return (get_exif_data(self.src_path)

sigal/utils.py

@@ -99,36 +99,21 @@ def call_subprocess(cmd):
 
 
 class cached_property(object):
-    '''Decorator for read-only properties evaluated only once.
-
-    © 2011 Christopher Arndt, MIT License
-    https://wiki.python.org/moin/PythonDecoratorLibrary#Cached_Properties
-
-    The value is cached in the '_cache' attribute of the object instance that
-    has the property getter method wrapped by this decorator. The '_cache'
-    attribute value is a dictionary which has a key for every property of the
-    object which is wrapped by this decorator. Each entry in the cache is
-    created only when the property is accessed for the first time and is a
-    two-element tuple with the last computed property value and the last time
-    it was updated in seconds since the epoch.
-
-    '''
-
-    def __call__(self, fget, doc=None):
-        self.fget = fget
-        self.__doc__ = doc or fget.__doc__
-        self.__name__ = fget.__name__
-        self.__module__ = fget.__module__
-        return self
-
-    def __get__(self, inst, owner):
-        try:
-            value = inst._cache[self.__name__]
-        except (KeyError, AttributeError):
-            value = self.fget(inst)
-            try:
-                cache = inst._cache
-            except AttributeError:
-                cache = inst._cache = {}
-            cache[self.__name__] = value
+    """ A property that is only computed once per instance and then replaces
+    itself with an ordinary attribute. Deleting the attribute resets the
+    property.
+    Source:
+        https://github.com/pydanny/cached-property (BSD Licensed)
+        https://github.com/bottlepy/bottle/commit/fa7733e075da0d790d809aa3d2f53071897e6f76
+
+    """
+
+    def __init__(self, func):
+        self.__doc__ = getattr(func, '__doc__')
+        self.func = func
+
+    def __get__(self, obj, cls):
+        if obj is None:
+            return self
+        value = obj.__dict__[self.func.__name__] = self.func(obj)
         return value