Use json to encode invalid values in configuration errors so that the user sees a proper repr of the value.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

tests/unit/config/config_test.py

@@ -552,7 +552,9 @@ class ConfigTest(unittest.TestCase):
             )
 
     def test_config_extra_hosts_list_of_dicts_validation_error(self):
-        expected_error_msg = "key 'extra_hosts' contains {'somehost': '162.242.195.82'}, which is an invalid type, it should be a string"
+        expected_error_msg = (
+            "key 'extra_hosts' contains {\"somehost\": \"162.242.195.82\"}, "
+            "which is an invalid type, it should be a string")
 
         with self.assertRaisesRegexp(ConfigurationError, expected_error_msg):
             config.load(

tests/unit/config/types_test.py

@@ -1,3 +1,6 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
 import pytest
 
 from compose.config.errors import ConfigurationError