Refactor selecting values from an attribute

Move the code to select particular keys/indices from an attribute value out
of the get_attr intrinsic functions and into the attributes module as a
separate function.

Change-Id: I16135159027cc52e8da74a4b5863fc8a8595d582

heat/engine/attributes.py

@@ -174,3 +174,27 @@ def __iter__(self):
     def __repr__(self):
         return ("Attributes for %s:\n\t" % self._resource_name +
                 '\n\t'.join(self._attributes.values()))
+
+
+def select_from_attribute(attribute_value, path):
+    '''
+    Select an element from an attribute value.
+
+    :param attribute_value: the attribute value.
+    :param path: a list of path components to select from the attribute.
+    :returns: the selected attribute component value.
+    '''
+    def get_path_component(collection, key):
+        if not isinstance(collection, (collections.Mapping,
+                                       collections.Sequence)):
+            raise TypeError(_("Can't traverse attribute path"))
+
+        if not isinstance(key, (basestring, int)):
+            raise TypeError(_('Path components in attributes must be strings'))
+
+        return collection[key]
+
+    try:
+        return reduce(get_path_component, path, attribute_value)
+    except (KeyError, IndexError, TypeError):
+        return None

heat/engine/hot/functions.py

@@ -14,6 +14,7 @@
 import collections
 
 from heat.common import exception
+from heat.engine import attributes
 from heat.engine.cfn import functions as cfn_funcs
 from heat.engine import function
 
@@ -116,22 +117,7 @@ def result(self):
             return None
 
         path_components = function.resolve(self._path_components)
-
-        def get_path_component(collection, key):
-            if not isinstance(collection, (collections.Mapping,
-                                           collections.Sequence)):
-                raise TypeError(_('"%s" can\'t traverse path') % self.fn_name)
-
-            if not isinstance(key, (basestring, int)):
-                raise TypeError(_('Path components in "%s" '
-                                  'must be strings') % self.fn_name)
-
-            return collection[key]
-
-        try:
-            return reduce(get_path_component, path_components, attribute)
-        except (KeyError, IndexError, TypeError):
-            return None
+        return attributes.select_from_attribute(attribute, path_components)
 
 
 class Replace(cfn_funcs.Replace):