class_docstr: Parameters
  ---------
  readers : dict[str -> SessionBarReader]
      A dict mapping country codes to SessionBarReader instances to
      service each country.
class_docstr_tok: ["Parameters","--","--","--","--","-","readers",":","dict","[","str","-",">","SessionBarReader","]","A","dict","mapping","country","codes","to","SessionBarReader","instances","to","service","each","country","."]
docstr: Returns
  -------
  dt : pd.Timestamp
      The first trading day (session) for which the reader can provide
      data.
docstr_tok: ["Returns","--","--","--","-","dt",":","pd.Timestamp","The","first","trading","day","(","session",")","for","which","the","reader","can","provide","data","."]
code: def first_trading_day(self):
      """
      Returns
      -------
      dt : pd.Timestamp
          The first trading day (session) for which the reader can provide
          data.
      """
      return min(
          reader.first_trading_day for reader in self._readers.values()
      )
  
imports: from functools import partial
  import h5py
  import logbook
  import numpy
  import pandas
  from six import iteritems, raise_from, viewkeys
  from six.moves import reduce
  from zipline.data.bar_reader import NoDataAfterDate, NoDataBeforeDate, NoDataForSid, NoDataOnDate
  from zipline.data.session_bars import CurrencyAwareSessionBarReader
  from zipline.utils.memoize import lazyval
  from zipline.utils.numpy_utils import bytes_array_to_native_str_object_array
  from zipline.utils.pandas_utils import check_indexes_all_same
label_desc: Use image node_id 9 for calling the MultiCountryDailyBarReader obj's underlying member method code with example usage: obj.first_trading_day() and returns: min