old_contents: # Copyright 2018 The dm_control Authors.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #    http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or  implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  # ============================================================================
  """Viewer's windowing systems."""
  
  from dm_control import render
  
  # pylint: disable=g-import-not-at-top
  # pylint: disable=invalid-name
  
  RenderWindow = None
  try:
    from dm_control.viewer.gui import glfw_gui
    RenderWindow = glfw_gui.GlfwWindow
  except ImportError:
    pass
  if not RenderWindow:
  
    def ErrorRenderWindow(*args, **kwargs):
      del args, kwargs
      raise ImportError(
          'Cannot create a window because no windowing system could be imported')
    RenderWindow = ErrorRenderWindow
  
  del render
  
new_contents: # Copyright 2018 The dm_control Authors.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #    http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or  implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  # ============================================================================
  """Viewer's windowing systems."""
  
  from dm_control import render
  
  # pylint: disable=g-import-not-at-top
  # pylint: disable=invalid-name
  
  RenderWindow = None
  
  if render.BACKEND == 'glfw':
    try:
      from dm_control.viewer.gui import glfw_gui
      RenderWindow = glfw_gui.GlfwWindow
    except ImportError:
      pass
  else:
  
    def ErrorRenderWindow(*args, **kwargs):
      del args, kwargs
      raise ImportError(
          'Cannot create a window because no windowing system could be imported')
    RenderWindow = ErrorRenderWindow
  
  del render
  
subject: Simplify import logic in `dm_control/viewer/gui/__init__.py`