Avoid globally modifying yaml library

The heat utils set up the yaml parser to always return unicode strings,
but are currently doing it in a way that infects all uses of the
library.

Make a subclass of the loader and run add_constructor on that so that
the new constructor is confined to the specific loader.

Change-Id: I49e97b5e2ae9b7862915ff83263718cf6cad32b8
Story: 2002040

openstack/cloud/_heat/template_format.py

@@ -18,23 +18,22 @@ if hasattr(yaml, 'CSafeLoader'):
 else:
     yaml_loader = yaml.SafeLoader
 
-if hasattr(yaml, 'CSafeDumper'):
-    yaml_dumper = yaml.CSafeDumper
-else:
-    yaml_dumper = yaml.SafeDumper
+
+class HeatYamlLoader(yaml_loader):
+    pass
 
 
 def _construct_yaml_str(self, node):
     # Override the default string handling function
     # to always return unicode objects
     return self.construct_scalar(node)
-yaml_loader.add_constructor(u'tag:yaml.org,2002:str', _construct_yaml_str)
+HeatYamlLoader.add_constructor(u'tag:yaml.org,2002:str', _construct_yaml_str)
 # Unquoted dates like 2013-05-23 in yaml files get loaded as objects of type
 # datetime.data which causes problems in API layer when being processed by
 # openstack.common.jsonutils. Therefore, make unicode string out of timestamps
 # until jsonutils can handle dates.
-yaml_loader.add_constructor(u'tag:yaml.org,2002:timestamp',
-                            _construct_yaml_str)
+HeatYamlLoader.add_constructor(
+    u'tag:yaml.org,2002:timestamp', _construct_yaml_str)
 
 
 def parse(tmpl_str):
@@ -49,7 +48,7 @@ def parse(tmpl_str):
         tpl = json.loads(tmpl_str)
     else:
         try:
-            tpl = yaml.load(tmpl_str, Loader=yaml_loader)
+            tpl = yaml.load(tmpl_str, Loader=HeatYamlLoader)
         except yaml.YAMLError:
             # NOTE(prazumovsky): we need to return more informative error for
             # user, so use SafeLoader, which return error message with template

releasenotes/notes/fix-yaml-load-3e6bd852afe549b4.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed an issue where importing openstacksdk changed the behavior of
+    ``yaml.load`` globally.