code: def handle(cls, value, **kwargs):
          
  
          try:
              env_var_name, default_val = value.split("::", 1)
          except ValueError:
              raise ValueError("Invalid value for default: %s. Must be in "
                               "<env_var>::<default value> format." % value)
  
          if env_var_name in kwargs['context'].environment:
              return kwargs['context'].environment[env_var_name]
          else:
              return default_val
docs: Use a value from the environment or fall back to a default if the
             environment doesn't contain the variable.
  
          Format of value:
  
              <env_var>::<default value>
  
          For example:
  
              Groups: ${default app_security_groups::sg-12345,sg-67890}
  
          If `app_security_groups` is defined in the environment, its defined
          value will be returned. Otherwise, `sg-12345,sg-67890` will be the
          returned value.
  
          This allows defaults to be set at the config file level.