code: def get_variable(self, variable_name, check_existence = False):
          
          variables = self.variables
          found = variables.get(variable_name)
          if not found and check_existence:
              raise VariableNotFound(variable_name, self)
          return found
docs: Get a variable from the tax and benefit system.
  
          :param variable_name: Name of the requested variable.
          :param check_existence: If True, raise an error if the requested variable does not exist.