Add jsonutils.dump_as_bytes() function for py3

The jsonutils.dumps() function returns bytes on Python 2 and Unicode
oon Python 3. In some cases, we always want bytes. For example, a
HTTP body must be bytes. This function avoids an condition call to
.encode() depending on the type or on the Python version.

For example:

    body = jsonutils.dumps(data)
    if isinstance(body, six.text_type):
        body = body.encode('utf-8')

can be replaced with:

    body = jsonutils.dump_as_bytes(data)

Change-Id: Ib9d8f1309982762b54d8a91b1f24f64d0ae6723a

oslo_serialization/jsonutils.py

@@ -179,12 +179,34 @@ def dumps(obj, default=to_primitive, **kwargs):
     :param kwargs: extra named parameters, please see documentation \
     of `json.dumps <https://docs.python.org/2/library/json.html#basic-usage>`_
     :returns: json formatted string
+
+    Use dump_as_bytes() to ensure that the result type is ``bytes`` on Python 2
+    and Python 3.
     """
     if is_simplejson:
         kwargs['namedtuple_as_object'] = False
     return json.dumps(obj, default=default, **kwargs)
 
 
+def dump_as_bytes(obj, default=to_primitive, encoding='utf-8', **kwargs):
+    """Serialize ``obj`` to a JSON formatted ``bytes``.
+
+    :param obj: object to be serialized
+    :param default: function that returns a serializable version of an object
+    :param encoding: encoding used to encode the serialized JSON output
+    :param kwargs: extra named parameters, please see documentation \
+    of `json.dumps <https://docs.python.org/2/library/json.html#basic-usage>`_
+    :returns: json formatted string
+
+    .. versionadded:: 2.0
+    """
+    serialized = dumps(obj, default=default, **kwargs)
+    if isinstance(serialized, six.text_type):
+        # On Python 3, json.dumps() returns Unicode
+        serialized = serialized.encode(encoding)
+    return serialized
+
+
 def dump(obj, fp, *args, **kwargs):
     """Serialize ``obj`` as a JSON formatted stream to ``fp``
 

oslo_serialization/tests/test_jsonutils.py

@@ -46,6 +46,9 @@ class JSONUtilsTestMixin(object):
     def test_dumps(self):
         self.assertEqual('{"a": "b"}', jsonutils.dumps({'a': 'b'}))
 
+    def test_dump_as_bytes(self):
+        self.assertEqual(b'{"a": "b"}', jsonutils.dump_as_bytes({'a': 'b'}))
+
     def test_dumps_namedtuple(self):
         n = collections.namedtuple("foo", "bar baz")(1, 2)
         self.assertEqual('[1, 2]', jsonutils.dumps(n))