code: def test_gb18030(self):
      s = io.BytesIO()
      c = codecs.getwriter("gb18030")(s)
      c.write("123")
      self.assertEqual(s.getvalue(), b"123")
      c.write("\U00012345")
      self.assertEqual(s.getvalue(), b"123\x907\x959")
      c.write("\uac00\u00ac")
      self.assertEqual(
          s.getvalue(), b"123\x907\x959\x827\xcf5\x810\x851"
      )
  
imports: import _multibytecodec
  import codecs
  import io
  import sys
  import textwrap
  import unittest
  from test import support
  from test.support import os_helper
  from test.support.os_helper import TESTFN
label_desc: Use image node_id 1 for calling the Test_StreamWriter obj's underlying member method code with example usage: obj.test_gb18030() without return types