code: def generate_df(self, exchange_name, freq, start, end):
      bundle = ExchangeBundle(exchange_name)
      index = bundle.get_calendar_periods_range(start, end, freq)
      df = pd.DataFrame(index=index, columns=self.columns)
      df.fillna(random.random(), inplace=True)
      return df
  
imports: import random
  import shutil
  import tempfile
  import pandas
  from nose.tools import assert_equals
  from catalyst.exchange.exchange_bcolz import BcolzExchangeBarWriter, BcolzExchangeBarReader
  from catalyst.exchange.exchange_bundle import ExchangeBundle
  from catalyst.exchange.utils.bundle_utils import get_df_from_arrays
label_desc: Use image node_id 4 for calling the TestBcolzWriter obj's underlying member method code with example usage: obj.generate_df(exchange_name, freq, start, end) and returns: df