code: def test_default_value_expansible_config(self):
      config_json = json.loads(
          """
              {
                  "task": {
                      "Task3": {
                          "model": {
                              "foobar": 1.0
                          }
                      }
                  },
                  "output": "foo/bar.pt"
              }
          """
      )
      config = config_from_json(PyTextConfig, config_json)
      self.assertTrue(isinstance(config.task, Task3))
      self.assertTrue(
          isinstance(config.task.model, ExpansibleModel.Config)
      )
      self.assertFalse(
          isinstance(config.task.model, AExpansibleModel.Config)
      )
      self.assertFalse(
          isinstance(config.task.model, BExpansibleModel.Config)
      )
      self.assertEqual(config.task.model.foobar, 1.0)
  
imports: import json
  import unittest
  from typing import List, Tuple, Union
  from pytext.config.component import Component, ComponentType
  from pytext.config.pytext_config import ConfigBase
  from pytext.config.serialize import config_from_json, config_to_json, ConfigParseError, MissingValueError, UnionTypeError
label_desc: Use image node_id 4 for calling the PytextConfigTest obj's underlying member method code with example usage: obj.test_default_value_expansible_config() without return types