class_docstr: Read a LMDB database and produce (k,v) raw bytes pairs.
  The raw bytes are usually not what you're interested in.
  You might want to use
  :class:`LMDBDataDecoder` or apply a
  mapper function after :class:`LMDBData`.
class_docstr_tok: ["Read","a","LMDB","database","and","produce","(","k",",","v",")","raw","bytes","pairs",".","The","raw","bytes","are","usually","not","what","you","'re","interested","in",".","You","might","want","to","use",":","class",":","`","LMDBDataDecoder","`","or","apply","a","mapper","function","after",":","class",":","`","LMDBData","`","."]
code: def __len__(self):
      return self._size
  
imports: import numpy
  import os
  import six
  from ..utils import logger
  from ..utils.argtools import log_once
  from ..utils.serialize import loads
  from ..utils.develop import create_dummy_class
  from ..utils.loadcaffe import get_caffe_pb
  from ..utils.timer import timed_operation
  from ..utils.utils import get_tqdm
  from .base import DataFlowReentrantGuard, RNGDataFlow
  from .common import MapData
label_desc: Use image node_id 6 for calling the LMDBData obj's underlying member method code with example usage: obj.__len__() and returns: self