code: def _transform(self, value):
          
          if self.transform:
              try:
                  value = self.transform(value)
              except:
                  import sys
                  exc_type, exc_obj, exc_tb = sys.exc_info()
                  raise AssertionError('Error applying transformation <{0}>: {2}: {3}'.format(
                      self.transform.__name__, value, exc_type.__name__, exc_obj))
  
          return value
docs: Applies any defined transformation to the given value