code: def test_optional(self):
      # Optional flags should have a default value
      with self.assertRaisesRegex(
          ValueError, "Optional flags must have a default value"
      ):
          flag_def.SingleValueFlagDef(
              name="value",
              parse_type=str,
              required=False,
          )
  
      opt_flag = flag_def.SingleValueFlagDef(
          name="value",
          parse_type=str,
          required=False,
          default_value="zero",
      )
  
      # Parser is able to parse a command line containing the optional argument.
      results = _new_parser(opt_flag).parse_args(["--value=forty-two"])
      self.assertEqual("forty-two", results.value)
  
      # Parser should return the default value if the command line does not
      # contain the optional argument.
      results = _new_parser(opt_flag).parse_args([])
      self.assertEqual("zero", results.value)
  
      # Parser should not accept flag without a value.
      with self.assertRaisesRegex(
          argument_parser.ParserError, "expected 1 argument"
      ):
          _new_parser(opt_flag).parse_args(["--value"])
  
imports: from __future__ import annotations
  import argparse
  import enum
  import math
  from absl.testing import absltest
  from google.generativeai.notebook import argument_parser
  from google.generativeai.notebook import flag_def
label_desc: Use image node_id 4 for calling the SingleValueFlagDefTest obj's underlying member method code with example usage: obj.test_optional() without return types