class_docstr: Class used to write data to an assets db.
  
  Parameters
  ----------
  engine : Engine or str
      An SQLAlchemy engine or path to a SQL database.
class_docstr_tok: ["Class","used","to","write","data","to","an","assets","db",".","Parameters","--","--","--","--","--","engine",":","Engine","or","str","An","SQLAlchemy","engine","or","path","to","a","SQL","database","."]
docstr: Returns a standard set of pandas.DataFrames:
  equities, futures, exchanges, root_symbols
docstr_tok: ["Returns","a","standard","set","of","pandas.DataFrames",":","equities",",","futures",",","exchanges",",","root_symbols"]
code: def _load_data(
      self,
      equities,
      futures,
      exchanges,
      root_symbols,
      equity_supplementary_mappings,
  ):
      """
      Returns a standard set of pandas.DataFrames:
      equities, futures, exchanges, root_symbols
      """
      # Set named identifier columns as indices, if provided.
      _normalize_index_columns_in_place(
          equities=equities,
          equity_supplementary_mappings=equity_supplementary_mappings,
          futures=futures,
          exchanges=exchanges,
          root_symbols=root_symbols,
      )
  
      futures_output = self._normalize_futures(futures)
  
      equity_supplementary_mappings_output = (
          self._normalize_equity_supplementary_mappings(
              equity_supplementary_mappings,
          )
      )
  
      exchanges_output = _generate_output_dataframe(
          data_subset=exchanges,
          defaults=_exchanges_defaults,
      )
  
      equities_output, equities_mappings = self._normalize_equities(
          equities,
          exchanges_output,
      )
  
      root_symbols_output = _generate_output_dataframe(
          data_subset=root_symbols,
          defaults=_root_symbols_defaults,
      )
  
      return AssetData(
          equities=equities_output,
          equities_mappings=equities_mappings,
          futures=futures_output,
          exchanges=exchanges_output,
          root_symbols=root_symbols_output,
          equity_supplementary_mappings=equity_supplementary_mappings_output,
      )
  
imports: from collections import namedtuple
  import re
  import numpy
  import pandas
  import sqlalchemy
  from toolz import first
  from zipline.errors import AssetDBVersionError
  from zipline.assets.asset_db_schema import ASSET_DB_VERSION, asset_db_table_names, asset_router, equities, equity_symbol_mappings, equity_supplementary_mappings, futures_contracts, exchanges, futures_root_symbols, metadata, version_info
  from zipline.utils.compat import ExitStack
  from zipline.utils.preprocess import preprocess
  from zipline.utils.range import from_tuple, intersecting_ranges
  from zipline.utils.sqlite_utils import coerce_string_to_eng
label_desc: Use image node_id 12 for calling the AssetDBWriter obj's underlying member method code with example usage: obj._load_data(equities, futures, exchanges, root_symbols, equity_supplementary_mappings) and returns: AssetData