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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _load_from_file(self, filename):
 """Load from a vocab file."""

    if not tf.gfile.Exists(filename):
      raise ValueError("File %s not found" % filename)
    with tf.gfile.Open(filename) as f:
      self._load_from_file_object(f)