code: def add_value_check(self, field_name, value_check,
                          code=VALUE_CHECK_FAILED,
                          message=MESSAGES[VALUE_CHECK_FAILED],
                          modulus=1):
          
  
          # guard conditions
          assert field_name in self._field_names, 'unexpected field name: %s' % field_name
          assert callable(value_check), 'value check must be a callable function'
  
          t = field_name, value_check, code, message, modulus
          self._value_checks.append(t)
docs: Add a value check function for the specified field.
  
          Arguments
          ---------
  
          `field_name` - the name of the field to attach the value check function
          to
  
          `value_check` - a function that accepts a single argument (a value) and
          raises a `ValueError` if the value is not valid
  
          `code` - problem code to report if a value is not valid, defaults to
          `VALUE_CHECK_FAILED`
  
          `message` - problem message to report if a value is not valid
  
          `modulus` - apply the check to every nth record, defaults to 1 (check
          every record)