code: def block_num(self):
      return len(self._block_manager.blocks)
  
imports: import numpy
  from .schema_manager import SchemaManager
  from .block_manager import BlockManager
  from .block_manager import BlockType
  import pandas
  from ..entity import types
  from typing import Union, List, Tuple
  from ..conf.default_config import DATAFRAME_BLOCK_ROW_SIZE
label_desc: Use image node_id 3 for calling the DataManager obj's underlying member method code with example usage: obj.block_num() and returns: len