new_contents: import urllib2
  import vcr
  
  class MockSerializer(object):
      def __init__(self):
          self.serialize_count = 0
          self.deserialize_count = 0
          self.load_args = None
  
      def deserialize(self, cassette_string):
          self.serialize_count += 1
          self.cassette_string = cassette_string
          return ([], [])
  
      def serialize(self, cassette_dict):
          self.deserialize_count += 1
          return ""
  
  def test_registered_serializer(tmpdir):
      ms = MockSerializer()
      my_vcr = vcr.VCR()
      my_vcr.register_serializer('mock', ms)
      tmpdir.join('test.mock').write('test_data')
      with my_vcr.use_cassette(str(tmpdir.join('test.mock')), serializer='mock'):
          urllib2.urlopen('http://httpbin.org/')
          assert ms.serialize_count == 1 # Serializer deserialized once
          assert ms.cassette_string == 'test_data' # and serialized the test data string
          assert ms.deserialize_count == 0 # and hasn't serialized yet
  
      assert ms.serialize_count == 1
  
  
  
  
subject: add tests for registering serializer