>>> g.V(vert).addE('related').to(vert_2).iterate()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/gremlin_python/process/traversal.py", line 57, in iterate
    try: self.nextTraverser()
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/gremlin_python/process/traversal.py", line 61, in nextTraverser
    self.traversal_strategies.apply_strategies(self)
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/gremlin_python/process/traversal.py", line 284, in apply_strategies
    traversal_strategy.apply(traversal)
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/gremlin_python/driver/remote_connection.py", line 95, in apply
    remote_traversal = self.remote_connection.submit(traversal.bytecode)
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/gremlin_python/driver/driver_remote_connection.py", line 53, in submit
    traversers = self._loop.run_sync(lambda: self.submit_traversal_bytecode(request_id, bytecode))
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/tornado/ioloop.py", line 457, in run_sync
    return future_cell[0].result()
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/tornado/concurrent.py", line 237, in result
    raise_exc_info(self._exc_info)
  File "<string>", line 3, in raise_exc_info
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/tornado/gen.py", line 285, in wrapper
    yielded = next(result)
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/gremlin_python/driver/driver_remote_connection.py", line 69, in submit_traversal_bytecode
    "gremlin": self._graphson_writer.writeObject(bytecode),
  File "/home/daidoji/python_homes/Analytics_Dev/lib64/python3.6/site-packages/gremlin_python/structure/io/graphson.py", line 72, in writeObject
    return json.dumps(self.toDict(objectData), separators=(',', ':'))
  File "/usr/lib64/python3.6/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "/usr/lib64/python3.6/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib64/python3.6/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib64/python3.6/json/encoder.py", line 180, in default
    o.__class__.__name__)
TypeError: Object of type 'Vertex' is not JSON serializable