Lazy import oslo_config as required

Openstackclient is taking the hit for importing oslo.config (which is
significant) just because we are doing an opportunistic load at import
time.

Change the opportunistic load to also happen at runtime to remove this
import. This has no user facing effect.

Change-Id: I3983a19fbe82cb1d360b5b5aec75b2697efa232d

keystoneauth1/loading/_utils.py

@@ -0,0 +1,40 @@
+# 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.
+
+cfg = None
+_NOT_FOUND = object()
+
+
+def get_oslo_config():
+    """Runtime load the oslo.config object.
+
+    In performance optimization of openstackclient it was determined that even
+    optimistically loading oslo.config if available had a performance cost.
+    Given that we used to only raise the ImportError when the function was
+    called also attempt to do the import to do everything at runtime.
+    """
+    global cfg
+
+    # First Call
+    if not cfg:
+        try:
+            from oslo_config import cfg
+        except ImportError:
+            cfg = _NOT_FOUND
+
+    if cfg is _NOT_FOUND:
+        raise ImportError("oslo.config is not an automatic dependency of "
+                          "keystoneauth. If you wish to use oslo.config "
+                          "you need to import it into your application's "
+                          "requirements file. ")
+
+    return cfg

keystoneauth1/loading/opts.py

@@ -15,10 +15,7 @@ import os
 
 from positional import positional
 
-try:
-    from oslo_config import cfg
-except ImportError:
-    cfg = None
+from keystoneauth1.loading import _utils
 
 
 __all__ = ('Opt',)
@@ -99,12 +96,7 @@ class Opt(object):
         return '<Opt: %s>' % self.name
 
     def _to_oslo_opt(self):
-        if not cfg:
-            raise ImportError("oslo.config is not an automatic dependency of "
-                              "keystoneauth. If you wish to use oslo.config "
-                              "you need to import it into your application's "
-                              "requirements file. ")
-
+        cfg = _utils.get_oslo_config()
         deprecated_opts = [cfg.DeprecatedOpt(o.name) for o in self.deprecated]
 
         return cfg.Opt(name=self.name,

keystoneauth1/loading/session.py

@@ -15,11 +15,7 @@ import os
 
 from positional import positional
 
-try:
-    from oslo_config import cfg
-except ImportError:
-    cfg = None
-
+from keystoneauth1.loading import _utils
 from keystoneauth1.loading import base
 from keystoneauth1 import session
 
@@ -160,11 +156,7 @@ class Session(base.BaseLoader):
 
         :returns: A list of oslo_config options.
         """
-        if not cfg:
-            raise ImportError("oslo.config is not an automatic dependency of "
-                              "keystoneauth. If you wish to use oslo.config "
-                              "you need to import it into your application's "
-                              "requirements file. ")
+        cfg = _utils.get_oslo_config()
 
         if deprecated_opts is None:
             deprecated_opts = {}
@@ -214,7 +206,7 @@ class Session(base.BaseLoader):
         :returns: The list of options that was registered.
         """
         opts = self.get_conf_options(deprecated_opts=deprecated_opts)
-        conf.register_group(cfg.OptGroup(group))
+        conf.register_group(_utils.get_oslo_config().OptGroup(group))
         conf.register_opts(opts, group=group)
         return opts