code: def __str__(self) -> str:
      res = ""
      if len(self._partitions) == 0:
          return "Empty Topo Graph."
  
      input_part = self.get_input_partition()
      if input_part is not None:
          res += "{\n"
          res += f"InputPartition:\n  partition_id={self._input_partition_id}\n{input_part}"
          res += "}\n"
  
      mid_parts = self.get_mid_partitions()
      for i, (partition_id, part) in enumerate(mid_parts.items()):
          res += "{\n"
          res += f"SubPartition_{i}:\n  partition_id={partition_id}\n  {part}"
          res += "}\n"
  
      output_part = self.get_output_partition()
      if output_part is not None:
          res += "{\n"
          res += f"OutputPartition:\n  partition_id={self._output_partition_id}\n{output_part}"
          res += "}\n"
  
      return res
  
imports: from dataclasses import dataclass
  from typing import Dict, List
label_desc: Use image node_id 12 for calling the Topo obj's underlying member method code with example usage: obj.__str__() and returns: res, str