class_docstr: Provides common methods for a protocol buffer object.
  
  Wraps a single string value. This makes testing equality easy.
code: def SerializeToString(self):  # pylint: disable=invalid-name
      # protobuf's SerializeToString returns binary string
      if six.PY3:
          return ("serialized:" + self.string).encode("utf-8")
      else:
          return "serialized:" + self.string
  
file_path: magenta/magenta/common/testing_lib.py
imports: import numpy
  import six
  from google.protobuf import text_format
label_desc: Use image node_id 3 for calling the MockStringProto obj's underlying member method code with example usage: obj.SerializeToString() and returns: unknown, unknown