Make client constructor optional

Turns out we know the mapping of service name to constsructor, so we can
try the import for the user without actually importing. Keep the
argument though, because this method should be usable by just about any
random openstack client lib. Also, because backwards compat.

Change-Id: I7e9672e3bf61b8b7b92d55903f4596382f18b515

README.rst

@@ -325,16 +325,16 @@ Constructing OpenStack Client objects
 
 If all you want to do is get a Client object from a python-*client library,
 and you want it to do all the normal things related to clouds.yaml, `OS_`
-environment variables, a hepler function is provided.
+environment variables, a hepler function is provided. The following
+will get you a fully configured `novaclient` instance.
 
 ::
 
   import argparse
 
-  from novaclient import client
   import os_client_config
 
-  nova = os_client_config.make_client('compute', client.Client)
+  nova = os_client_config.make_client('compute')
 
 If you want to do the same thing but also support command line parsing.
 
@@ -342,11 +342,10 @@ If you want to do the same thing but also support command line parsing.
 
   import argparse
 
-  from novaclient import client
   import os_client_config
 
   nova = os_client_config.make_client(
-      'compute', client.Client, options=argparse.ArgumentParser())
+      'compute', options=argparse.ArgumentParser())
 
 If you want to get fancier than that in your python, then the rest of the
 API is avaiable to you. But often times, you just want to do the one thing.

os_client_config/cloud_config.py

@@ -12,6 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import importlib
 import warnings
 
 from keystoneauth1 import adapter
@@ -20,9 +21,44 @@ from keystoneauth1 import session
 import requestsexceptions
 
 from os_client_config import _log
+from os_client_config import constructors
 from os_client_config import exceptions
 
 
+def _get_client(service_key):
+    class_mapping = constructors.get_constructor_mapping()
+    if service_key not in class_mapping:
+        raise exceptions.OpenStackConfigException(
+            "Service {service_key} is unkown. Please pass in a client"
+            " constructor or submit a patch to os-client-config".format(
+                service_key=service_key))
+    mod_name, ctr_name = class_mapping[service_key].rsplit('.', 1)
+    lib_name = mod_name.split('.')[0]
+    try:
+        mod = importlib.import_module(mod_name)
+    except ImportError:
+        raise exceptions.OpenStackConfigException(
+            "Client for '{service_key}' was requested, but"
+            " {mod_name} was unable to be imported. Either import"
+            " the module yourself and pass the constructor in as an argument,"
+            " or perhaps you do not have python-{lib_name} installed.".format(
+                service_key=service_key,
+                mod_name=mod_name,
+                lib_name=lib_name))
+    try:
+        ctr = getattr(mod, ctr_name)
+    except AttributeError:
+        raise exceptions.OpenStackConfigException(
+            "Client for '{service_key}' was requested, but although"
+            " {mod_name} imported fine, the constructor at {fullname}"
+            " as not found. Please check your installation, we have no"
+            " clue what is wrong with your computer.".format(
+                service_key=service_key,
+                mod_name=mod_name,
+                fullname=class_mapping[service_key]))
+    return ctr
+
+
 def _make_key(key, service_type):
     if not service_type:
         return key
@@ -217,7 +253,7 @@ class CloudConfig(object):
         return endpoint
 
     def get_legacy_client(
-            self, service_key, client_class, interface_key=None,
+            self, service_key, client_class=None, interface_key=None,
             pass_version_arg=True, **kwargs):
         """Return a legacy OpenStack client object for the given config.
 
@@ -254,6 +290,9 @@ class CloudConfig(object):
                        Client constructor, so this is in case anything
                        additional needs to be passed in.
         """
+        if not client_class:
+            client_class = _get_client(service_key)
+
         # Because of course swift is different
         if service_key == 'object-store':
             return self._get_swift_client(client_class=client_class, **kwargs)

os_client_config/constructors.py

@@ -0,0 +1,28 @@
+# Copyright (c) 2014 Hewlett-Packard Development Company, L.P.
+#
+# 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.
+
+import json
+import os
+
+_json_path = os.path.join(
+    os.path.dirname(os.path.realpath(__file__)), 'constructors.json')
+_class_mapping = None
+
+
+def get_constructor_mapping():
+    global _class_mapping
+    if not _class_mapping:
+        with open(_json_path, 'r') as json_file:
+            _class_mapping = json.load(json_file)
+    return _class_mapping

os_client_config/constructos.json

@@ -0,0 +1,10 @@
+{
+    "compute": "novaclient.client.Client",
+    "database": "troveclient.client.Client",
+    "identity": "keystoneclient.client.Client",
+    "image": "glanceclient.Client",
+    "network": "neutronclient.neutron.client.Client",
+    "object-store": "swiftclient.client.Connection",
+    "orchestration": "heatclient.client.Client",
+    "volume": "cinderclient.client.Client"
+}