In keep, make environment variables the defaults of several args

keep

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 
 import argparse
+import os
 
 from barbicanclient import client
 
@@ -23,15 +24,23 @@ class Keep:
                             choices=["order", "secret"],
                             help="type to operate on")
         parser.add_argument('--auth_endpoint', '-A',
-                            help='the URL to authenticate against')
-        parser.add_argument('--user', '-U', help='the user to authenticate as')
-        parser.add_argument('--password', '-P',
-                            help='the API key or password to '
-                            'authenticate with')
-        parser.add_argument('--tenant', '-T', help='the tenant ID')
-        parser.add_argument('--endpoint', '-E',
-                            help='the URL of the barbican server')
-        parser.add_argument('--token', '-K', help='the authentication token')
+                            default=env('OS_AUTH_URL'),
+                            help='the URL to authenticate against (default: '
+                                 '%(default)s)')
+        parser.add_argument('--user', '-U', default=env('OS_USERNAME'),
+                            help='the user to authenticate as (default: %(de'
+                                 'fault)s)')
+        parser.add_argument('--password', '-P', default=env('OS_PASSWORD'),
+                            help='the API key or password to authenticate with'
+                            ' (default: %(default)s)')
+        parser.add_argument('--tenant', '-T', default=env('OS_TENANT_NAME'),
+                            help='the tenant ID (default: %(default)s)')
+        parser.add_argument('--endpoint', '-E', default=env('SERVICE_ENDPOINT')
+                            , help='the URL of the barbican server (default: %'
+                            '(default)s)')
+        parser.add_argument('--token', '-K', default=env('SERVICE_TOKEN'),
+                            help='the authentication token (default: %(default'
+                            ')s)')
         return parser
 
     def add_create_args(self):
@@ -152,6 +161,20 @@ class Keep:
         args.func(args)
 
 
+def env(*vars, **kwargs):
+    """Search for the first defined of possibly many env vars
+
+    Returns the first environment variable defined in vars, or
+    returns the default defined in kwargs.
+
+    """
+    for v in vars:
+        value = os.environ.get(v, None)
+        if value:
+            return value
+    return kwargs.get('default', '')
+
+
 def main():
     k = Keep()
     k.execute()