code: def test_jax_platforms_flag(self):
      self._register_factory(
          "platform_A", 20, assert_used_at_most_once=True
      )
      self._register_factory(
          "platform_B", 10, assert_used_at_most_once=True
      )
  
      with config.jax_platforms("cpu,platform_A"):
          backend = xb.get_backend()
          self.assertEqual(backend.platform, "cpu")
          # Only specified backends initialized.
          self.assertEqual(len(xb._backends), 2)
  
          backend = xb.get_backend("platform_A")
          self.assertEqual(backend.platform, "platform_A")
  
          with self.assertRaisesRegex(
              RuntimeError, "Unknown backend platform_B"
          ):
              backend = xb.get_backend("platform_B")
  
imports: import os
  import platform
  import time
  import warnings
  from absl import logging
  from absl.testing import absltest
  from jax._src import compiler
  from jax._src import config
  from jax._src import test_util
  from jax._src import xla_bridge
  from jax._src.interpreters import xla
  from jax._src.lib import xla_client
label_desc: Use image node_id 14 for calling the GetBackendTest obj's underlying member method code with example usage: obj.test_jax_platforms_flag() without return types