docs: def get_parents(self):
          """
          Returns the parents of the variables present in the network
  
          Example
          --------
          >>> from pgmpy.readwrite import BIFReader
          >>> reader = BIFReader("bif_test.bif")
          >>> reader.get_parents()
          {'bowel-problem': [],
          'dog-out': ['family-out', 'bowel-problem'],
          'family-out': [],
          'hear-bark': ['dog-out'],
          'light-on': ['family-out']}
          """
          variable_parents = {}
          for block in self.probability_block():
              names = self.probability_expr.searchString(block.split('\n')[0])[0]
              variable_parents[names[0]] = names[1:]
          return variable_parents