Raise PluginNotFound exception when not found

A new exception class :class:`.exception.PluginNotFound` is now
raised when a particular cache plugin class cannot be located
either as a setuptools entrypoint or as a registered backend.
Previously, a plain ``Exception`` was thrown.  Pull request
courtesy Jamie Lennox.

Change-Id: Iceab2d613d2d802618dc0bed8ec2db694176c788

docs/build/api.rst

@@ -41,6 +41,12 @@ Backends
 .. automodule:: dogpile.cache.backends.null
     :members:
 
+Exceptions
+==========
+
+.. automodule:: dogpile.cache.exception
+    :members:
+
 Plugins
 ========
 

docs/build/changelog.rst

@@ -22,6 +22,15 @@ Changelog
       ends up being used as a cache key do not create multiple values.
       Pull request courtesy Paul Brown.
 
+    .. change::
+      :tags: bug
+
+      A new exception class :class:`.exception.PluginNotFound` is now
+      raised when a particular cache plugin class cannot be located
+      either as a setuptools entrypoint or as a registered backend.
+      Previously, a plain ``Exception`` was thrown.  Pull request
+      courtesy Jamie Lennox.
+
 .. changelog::
     :version: 0.6.3
     :released: Thu May 18, 2017

dogpile/cache/exception.py

@@ -15,3 +15,11 @@ class RegionNotConfigured(DogpileCacheException):
 
 class ValidationError(DogpileCacheException):
     """Error validating a value or option."""
+
+
+class PluginNotFound(DogpileCacheException):
+    """The specified plugin could not be found.
+
+    .. versionadded:: 0.6.4
+
+    """

dogpile/cache/region.py

@@ -410,7 +410,13 @@ class CacheRegion(object):
                 "configured with backend: %s.  "
                 "Specify replace_existing_backend=True to replace."
                 % self.backend)
-        backend_cls = _backend_loader.load(backend)
+
+        try:
+            backend_cls = _backend_loader.load(backend)
+        except PluginLoader.NotFound:
+            raise exception.PluginNotFound(
+                "Couldn't find cache plugin to load: %s" % backend)
+
         if _config_argument_dict:
             self.backend = backend_cls.from_config_dict(
                 _config_argument_dict,

dogpile/util/__init__.py

@@ -1,4 +1,4 @@
 from .nameregistry import NameRegistry  # noqa
 from .readwrite_lock import ReadWriteMutex  # noqa
 from .langhelpers import PluginLoader, memoized_property, \
-    coerce_string_conf, to_list, KeyReentrantMutex  # noqa
+    coerce_string_conf, to_list, KeyReentrantMutex  # noqa

dogpile/util/langhelpers.py

@@ -39,9 +39,9 @@ class PluginLoader(object):
                 self.impls[name] = impl.load
                 return impl.load()
             else:
-                raise Exception(
-                    "Can't load plugin %s %s" %
-                    (self.group, name))
+                raise self.NotFound(
+                    "Can't load plugin %s %s" % (self.group, name)
+                )
 
     def register(self, name, modulepath, objname):
         def load():
@@ -49,6 +49,9 @@ class PluginLoader(object):
             return getattr(mod, objname)
         self.impls[name] = load
 
+    class NotFound(Exception):
+        """The specified plugin could not be found."""
+
 
 class memoized_property(object):
     """A read-only @property that is only evaluated once."""

tests/cache/test_region.py

@@ -152,6 +152,15 @@ class RegionTest(TestCase):
         )
         eq_(reg.is_configured, False)
 
+    def test_invalid_backend(self):
+        reg = CacheRegion()
+        assert_raises_message(
+            exception.PluginNotFound,
+            "Couldn't find cache plugin to load: unknown",
+            reg.configure, 'unknown'
+        )
+        eq_(reg.is_configured, False)
+
     def test_set_get_value(self):
         reg = self._region()
         reg.set("some key", "some value")