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","."]
code: def _country_code_for_assets(self, assets):
      country_codes = self._country_map.get(assets)
  
      # In some versions of pandas (observed in 0.22), Series.get()
      # returns None if none of the labels are in the index.
      if country_codes is not None:
          unique_country_codes = country_codes.dropna().unique()
          num_countries = len(unique_country_codes)
      else:
          num_countries = 0
  
      if num_countries == 0:
          raise ValueError("At least one valid asset id is required.")
      elif num_countries > 1:
          raise NotImplementedError(
              (
                  "Assets were requested from multiple countries ({}),"
                  " but multi-country reads are not yet supported."
              ).format(list(unique_country_codes))
          )
  
      return np.asscalar(unique_country_codes)
  
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 5 for calling the MultiCountryDailyBarReader obj's underlying member method code with example usage: obj._country_code_for_assets(assets) and returns: np