code: def DF_calc(classes):
      """
      Calculate Chi-squared degree of freedom (DF).
  
      :param classes: confusion matrix classes
      :type classes: list
      :return: DF as int
      """
      try:
          return (len(classes) - 1) ** 2
      except Exception:
          return "None"
  
imports: from __future__ import division
  import math
  import operator
  from functools import reduce
  from .pycm_interpret import *
  from .pycm_ci import kappa_SE_calc, CI_calc, SE_calc
  from .pycm_util import complement
label_desc: Use image node_id 34 for calling a global function with example usage: DF_calc(classes) and returns: unknown, str