class_docstr: Dataset for image detection.
  
  Args:
     transform(callmethod) : The method of preprocess images.
     mode(str): The mode for preparing dataset.
  
  Returns:
      DataSet: An iterable object for data iterating
class_docstr_tok: ["Dataset","for","image","detection",".","Args",":","transform","(","callmethod",")",":","The","method","of","preprocess","images",".","mode","(","str",")",":","The","mode","for","preparing","dataset",".","Returns",":","DataSet",":","An","iterable","object","for","data","iterating"]
code: def __len__(self):
      return len(self.data)
  
imports: import os
  import copy
  from typing import Callable
  import paddle
  import numpy
  from paddlehub.env import DATA_HOME
  from pycocotools.coco import COCO
label_desc: Use image node_id 3 for calling the DetectionData obj's underlying member method code with example usage: obj.__len__() and returns: len