code: def assert_month(self, start_month, actual_end_month):
      if start_month == 1:
          expected_end_month = 12
      else:
          expected_end_month = start_month - 1
  
      self.assertEqual(expected_end_month, actual_end_month)
  
imports: import datetime
  import calendar
  import pandas
  import numpy
  import catalyst.finance.risk
  from catalyst.utils import factory
  from catalyst.finance.trading import SimulationParameters
  from catalyst.testing.fixtures import WithTradingEnvironment, CatalystTestCase
  from catalyst.finance.risk.period import RiskMetricsPeriod
label_desc: Use image node_id 20 for calling the TestRisk obj's underlying member method code with example usage: obj.assert_month(start_month, actual_end_month) without return types