'Persist' __getattr__ results

This patch adds a new decorator that helps creating 'cachable'
__getattr__ methods. The decorator sets the returned value as an
attribute of the instance.

The patch also decorates `Pipeline`'s __getattr__ method

Implements blueprint storage-pipeline

Change-Id: I6b052d536d5c6e6edaeec884b3c4a0e8703b1036

marconi/common/decorators.py

@@ -13,6 +13,32 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import functools
+
+
+def cached_getattr(meth):
+    """Caches attributes returned by __getattr__
+
+    It can be used to cache results from
+    __getattr__ and reduce the debt of calling
+    it again when the same attribute is accessed.
+
+    This decorator caches attributes by setting
+    them in the object itself.
+
+    The wrapper returned by this decorator won't alter
+    the returned value.
+
+    :returns: A wrapper around the decorated method.
+    """
+
+    @functools.wraps(meth)
+    def wrapper(self, method_name):
+        attr = meth(self, method_name)
+        setattr(self, method_name, attr)
+        return attr
+    return wrapper
+
 
 def lazy_property(write=False, delete=True):
     """Creates a lazy property.

marconi/common/pipeline.py

@@ -34,6 +34,7 @@ import functools
 
 import six
 
+from marconi.common import decorators
 import marconi.openstack.common.log as logging
 
 LOG = logging.getLogger(__name__)
@@ -47,6 +48,7 @@ class Pipeline(object):
     def append(self, stage):
         self._pipeline.append(stage)
 
+    @decorators.cached_getattr
     def __getattr__(self, name):
         return functools.partial(self.consume_for, name)
 

tests/unit/common/test_decorators.py

@@ -0,0 +1,33 @@
+# Copyright (c) 2013 Red Hat, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from marconi.common import decorators
+from marconi.tests import base
+
+
+class TestDecorators(base.TestBase):
+
+    def test_cached_getattr(self):
+
+        class TestClass(object):
+
+            @decorators.cached_getattr
+            def __getattr__(self, name):
+                return name
+
+        instance = TestClass()
+        result = instance.testing
+        self.assertEqual(result, 'testing')
+        self.assertIn('testing', instance.__dict__)