def __init__(self, filename=None):
    """Initialize and read from a file, if provided.

    Args:
      filename: filename from which to read vocab. If None, do not load a
        vocab
    """
    self._alphabet = set()
    self.filename = filename
    if filename is not None:
      self._load_from_file(filename)
    super(SubwordTextEncoder, self).__init__()
Initialize and read from a file, if provided.

    Args:
      filename: filename from which to read vocab. If None, do not load a
        vocab
    
tensor2tensor/data_generators/text_encoder.py