Parse JSON API errors

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

docker/errors.py

@@ -11,7 +11,10 @@ class APIError(requests.exceptions.HTTPError):
         self.explanation = explanation
 
         if self.explanation is None and response.content:
-            self.explanation = response.content.strip()
+            try:
+                self.explanation = response.json()['message']
+            except ValueError:
+                self.explanation = response.content.strip()
 
     def __str__(self):
         message = super(APIError, self).__str__()

tests/integration/container_test.py

@@ -118,7 +118,7 @@ class CreateContainerTest(helpers.BaseTestCase):
         self.client.wait(id)
         with self.assertRaises(docker.errors.APIError) as exc:
             self.client.remove_container(id)
-        err = exc.exception.response.text
+        err = exc.exception.explanation
         self.assertIn(
             'You cannot remove a running container', err
         )
@@ -289,7 +289,7 @@ class CreateContainerTest(helpers.BaseTestCase):
             )
             self.client.start(container)
 
-        assert six.b(expected_msg) in excinfo.value.explanation
+        assert excinfo.value.explanation == expected_msg
 
     def test_valid_no_log_driver_specified(self):
         log_config = docker.utils.LogConfig(

tests/integration/errors_test.py

+from docker.errors import APIError
+from .. import helpers
+
+
+class ErrorsTest(helpers.BaseTestCase):
+    def test_api_error_parses_json(self):
+        container = self.client.create_container(
+            helpers.BUSYBOX,
+            ['sleep', '10']
+        )
+        self.client.start(container['Id'])
+        with self.assertRaises(APIError) as cm:
+            self.client.remove_container(container['Id'])
+        explanation = cm.exception.explanation
+        assert 'You cannot remove a running container' in explanation
+        assert '{"message":' not in explanation
+        self.client.remove_container(container['Id'], force=True)