class_docstr: A dataset wrapping over a RecordIO (.rec) file.
  
  Each sample is a string representing the raw content of an record.
  
  Parameters
  ----------
  filename : str
      Path to rec file.
code: def __len__(self):
      return len(self._record.keys)
  
imports: import os
  from ...None import recordio, ndarray
  from ...util import default_array
label_desc: Use image node_id 3 for calling the RecordFileDataset obj's underlying member method code with example usage: obj.__len__() and returns: len