Handle unicode error in json serialize properly.

tests/integration/test_basic.py

@@ -1,4 +1,4 @@
-'''Basic tests about cassettes'''
+'''Basic tests for cassettes'''
 # coding=utf-8
 
 # External imports

tests/unit/test_serialize.py

@@ -1,21 +1,34 @@
+import mock
 import pytest
+
 from vcr.serialize import deserialize
 from vcr.serializers import yamlserializer, jsonserializer
 
+
 def test_deserialize_old_yaml_cassette():
     with open('tests/fixtures/migration/old_cassette.yaml', 'r') as f:
         with pytest.raises(ValueError):
             deserialize(f.read(), yamlserializer)
 
+
 def test_deserialize_old_json_cassette():
     with open('tests/fixtures/migration/old_cassette.json', 'r') as f:
         with pytest.raises(ValueError):
             deserialize(f.read(), jsonserializer)
 
+
 def test_deserialize_new_yaml_cassette():
     with open('tests/fixtures/migration/new_cassette.yaml', 'r') as f:
         deserialize(f.read(), yamlserializer)
 
+
 def test_deserialize_new_json_cassette():
     with open('tests/fixtures/migration/new_cassette.json', 'r') as f:
         deserialize(f.read(), jsonserializer)
+
+
+@mock.patch.object(jsonserializer.json, 'dumps',
+                   side_effect=UnicodeDecodeError('utf-8', b'unicode error in serialization',
+                                                  0, 10, 'blew up'))
+def test_serialize_constructs_UnicodeDecodeError(mock_dumps):
+    jsonserializer.serialize({})

vcr/serializers/jsonserializer.py

@@ -11,10 +11,14 @@ def deserialize(cassette_string):
 def serialize(cassette_dict):
     try:
         return json.dumps(cassette_dict, indent=4)
-    except UnicodeDecodeError:
+    except UnicodeDecodeError as original:
         raise UnicodeDecodeError(
-            "Error serializing cassette to JSON. ",
+            original.encoding,
-            "Does this HTTP interaction contain binary data? ",
+            b"Error serializing cassette to JSON",
-            "If so, use a different serializer (like the yaml serializer) ",
+            original.start,
-            "for this request"
+            original.end,
+            original.message +
+            ("Does this HTTP interaction contain binary data? "
+             "If so, use a different serializer (like the yaml serializer) "
+             "for this request?")
         )