class_docstr: Text span index class.
  Holds TF and DF values per span. Text spans are normalized and similar
  spans are mapped to the same TF DF values.
class_docstr_tok: ["Text","span","index","class",".","Holds","TF","and","DF","values","per","span",".","Text","spans","are","normalized","and","similar","spans","are","mapped","to","the","same","TF","DF","values","."]
docstr: Get DF of phrase in corpus
docstr_tok: ["Get","DF","of","phrase","in","corpus"]
code: def df(self, phrase):
      """
      Get DF of phrase in corpus
      """
      pid = CorpusIndex.get_pid(phrase)
      if self.df_index.get(pid, None) is not None:
          return len(self.df_index[pid])
      return 0
  
imports: import itertools
  import math
  import numpy
  import logging
  from spacy.tokens.doc import Doc
  from spacy.tokens.span import Span
  from spacy.tokens.token import Token
label_desc: Use image node_id 9 for calling the CorpusIndex obj's underlying member method code with example usage: obj.df(phrase) and returns: int, len