class_docstr: An RpcController mediates a single method call.
  
  The primary purpose of the controller is to provide a way to manipulate
  settings specific to the RPC implementation and to find out about RPC-level
  errors. The methods provided by the RpcController interface are intended
  to be a "least common denominator" set of features which we expect all
  implementations to support.  Specific implementations may provide more
  advanced features (e.g. deadline propagation).
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docstr: Initiate cancellation.
  
  Advises the RPC system that the caller desires that the RPC call be
  canceled.  The RPC system may cancel it immediately, may wait awhile and
  then cancel it, or may not even cancel the call at all.  If the call is
  canceled, the "done" callback will still be called and the RpcController
  will indicate that the call failed at that time.
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code: def StartCancel(self):
      """Initiate cancellation.
  
      Advises the RPC system that the caller desires that the RPC call be
      canceled.  The RPC system may cancel it immediately, may wait awhile and
      then cancel it, or may not even cancel the call at all.  If the call is
      canceled, the "done" callback will still be called and the RpcController
      will indicate that the call failed at that time.
      """
      raise NotImplementedError
  
file_path: turicreate/src/external/coremltools_wrap/coremltools/deps/protobuf/python/google/protobuf/service.py
label_desc: Use image node_id 4 for calling the RpcController obj's underlying member method code with example usage: obj.StartCancel() without return types