class_docstr: docstring for SeededFakeDataFlow
class_docstr_tok: ["docstring","for","SeededFakeDataFlow"]
code: def __len__(self):
      return self._size
  
file_path: tensorpack/tensorpack/dataflow/serialize_test.py
imports: import tempfile
  import numpy
  import os
  import unittest
  from tensorpack.dataflow import HDF5Serializer, LMDBSerializer, NumpySerializer, TFRecordSerializer
  from tensorpack.dataflow.base import DataFlow
label_desc: Use image node_id 3 for calling the SeededFakeDataFlow obj's underlying member method code with example usage: obj.__len__() and returns: self