class_docstr: Add Prefix to src (& tgt) sentence.
docstr: Get prefix string of a `corpus`.
docstr_tok: ["Get","prefix","string","of","a","`","corpus","`","."]
code: def _get_prefix(corpus):
      """Get prefix string of a `corpus`."""
      if "prefix" in corpus["transforms"]:
          src_prefix = corpus.get("src_prefix", "")
          tgt_prefix = corpus.get("tgt_prefix", "")
          prefix = {"src": src_prefix, "tgt": tgt_prefix}
      else:
          prefix = None
      return prefix
  
imports: from onmt.utils.logging import logger
  from onmt.transforms import register_transform
  from .transform import Transform, ObservableStats
label_desc: Use image node_id 3 for calling the PrefixTransform obj's underlying member method code with example usage: obj._get_prefix(corpus) and returns: prefix