Add _single_path method for single resources path

It makes sure that resource id is not None, so that e.g. DELETE request
is not accidentally called on a collection URL.

Also _path method stub is now in base.Manager, to be overriden by child
classes.

tuskarclient/common/base.py

@@ -47,6 +47,26 @@ class Manager(object):
     def __init__(self, api):
         self.api = api
 
+    @staticmethod
+    def _path(id=None):
+        """Helper method to be defined in subclasses. It returns the
+        resource/collection path. If id is given, then single resource
+        path is returned. Otherwise the collection path is returned.
+
+        :param id: id of the resource (optional)
+        """
+        raise NotImplementedError("_path method not implemented.")
+
+    def _single_path(self, id):
+        """This is like the _path method, but it asserts that the rack_id
+        parameter is not None. This is useful e.g. when you want to make sure
+        that you can't issue a DELETE request on a collection URL.
+        """
+        if not id:
+            raise ValueError("{0} id for deletion must not be null."
+                             .format(self.resource_class))
+        return self._path(id)
+
     def _create(self, url, body):
         resp, body = self.api.json_request('POST', url, body=body)
         if body:

tuskarclient/tests/common/test_base.py

@@ -42,3 +42,14 @@ class ManagerTest(tutils.TestCase):
         self.m._list.assert_called_with('url', response_key='response_key',
                                         obj_class='obj_class',
                                         body='body', expect_single=True)
+
+    def test_path(self):
+        self.assertRaises(NotImplementedError, self.m._path)
+
+    def test_single_path(self):
+        self.m._path = mock.Mock(return_value='/v1/somethings/42')
+        self.m._single_path(42)
+        self.m._path.assert_called_with(42)
+
+    def test_single_path_without_id(self):
+        self.assertRaises(ValueError, self.m._single_path, None)