code: def __str__(self, level=0):
      ret = ""
      for child in self.children:
          if isinstance(child, type(self)):
              ret += child.__str__(level + 1)
          else:
              ret += "\t" * level + str(child) + "\n"
      return ret
  
imports: from operator import itemgetter
  import numpy
label_desc: Use image node_id 2 for calling the Tree obj's underlying member method code with example usage: obj.__str__(level) and returns: ret