code: def _ts_to_obj(
      df: pd.DataFrame, historical_columns: list
  ) -> pd.DataFrame:
      """
      Casts all historical columns in a dataframe to `object` type.
  
      :param df: Input dataframe
      :param historical_columns: Historical columns to type cast
  
      :return: Dataframe with `object`-typed historical columns
      """
      for hist_col in historical_columns:
          df.loc[:, hist_col] = df[hist_col].astype(object)
      return df
  
file_path: lightwood/lightwood/data/timeseries_transform.py
imports: from typing import Dict, Optional
  from functools import partial
  import multiprocessing
  import numpy
  import pandas
  from lightwood.helpers.parallelism import get_nr_procs
  from type_infer.dtype import dtype
  from lightwood.api.types import TimeseriesSettings, PredictionArguments
  from lightwood.helpers.log import log
label_desc: Use image node_id 3 for calling a global function with example usage: _ts_to_obj(df, historical_columns) and returns: df