code: def _to_string(data):
      
      sdata = data.copy()
      for k, v in data.items():
          if isinstance(v, datetime):
              sdata[k] = timestamp_to_date_str(v)
  
          elif not isinstance(v, (string_types, float, int)):
              sdata[k] = str(v)
  
      return sdata
docs: Convert to string all values in `data`.
  
      Parameters
      ----------
      data: dict[str]->object
  
      Returns
      -------
      string_data: dict[str]->str