old_contents: #!/usr/bin/env python
  
  # Copyright 2016 Google Inc. All Rights Reserved.
  #
  # 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.
  
  """Example of using the Compute Engine API to watch for maintenance notices.
  
  For more information, see the README.md under /compute.
  """
  
  # [START all]
  
  import time
  
  import requests
  
  
  METADATA_URL = 'http://metadata.google.internal/computeMetadata/v1/'
  METADATA_HEADERS = {'Metadata-Flavor': 'Google'}
  
  
  def wait_for_maintenance(callback):
      url = METADATA_URL + 'instance/maintenance-event'
      last_maintenance_event = None
      # [START hanging_get]
      last_etag = '0'
  
      while True:
          r = requests.get(
              url,
              params={'last_etag': last_etag, 'wait_for_change': True},
              headers=METADATA_HEADERS)
  
          # During maintenance the service can return a 503, so these should
          # be retried.
          if r.status_code == 503:
              time.sleep(1)
              continue
          r.raise_for_status()
  
          last_etag = r.headers['etag']
          # [END hanging_get]
  
          if r.text == 'NONE':
              maintenance_event = None
          else:
              # Possible events:
              #   MIGRATE_ON_HOST_MAINTENANCE: instance will be migrated
              #   TERMINATE_ON_HOST_MAINTENANCE: instance will be shut down
              maintenance_event = r.text
  
          if maintenance_event != last_maintenance_event:
              last_maintenance_event = maintenance_event
              callback(maintenance_event)
  
  
  def maintenance_callback(event):
      if event:
          print('Undergoing host maintenance: {}'.format(event))
      else:
          print('Finished host maintenance')
  
  
  def main():
      wait_for_maintenance(maintenance_callback)
  
  
  if __name__ == '__main__':
      main()
  # [END all]
  
new_contents: #!/usr/bin/env python
  
  # Copyright 2016 Google Inc. All Rights Reserved.
  #
  # 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.
  
  """Example of using the Compute Engine API to watch for maintenance notices.
  
  For more information, see the README.md under /compute.
  """
  
  # [START all]
  
  import time
  
  import requests
  
  
  METADATA_URL = 'http://metadata.google.internal/computeMetadata/v1/'
  METADATA_HEADERS = {'Metadata-Flavor': 'Google'}
  
  
  def wait_for_maintenance(callback):
      url = METADATA_URL + 'instance/maintenance-event'
      last_maintenance_event = None
      # [START hanging_get]
      last_etag = '0'
  
      while True:
          r = requests.get(
              url,
              params={'last_etag': last_etag, 'wait_for_change': True},
              headers=METADATA_HEADERS)
  
          # During maintenance the service can return a 503, so these should
          # be retried.
          if r.status_code == 503:
              time.sleep(1)
              continue
          r.raise_for_status()
  
          last_etag = r.headers['etag']
          # [END hanging_get]
  
          if r.text == 'NONE':
              maintenance_event = None
          else:
              maintenance_event = r.text
  
          if maintenance_event != last_maintenance_event:
              last_maintenance_event = maintenance_event
              callback(maintenance_event)
  
  
  def maintenance_callback(event):
      if event:
          print('Undergoing host maintenance: {}'.format(event))
      else:
          print('Finished host maintenance')
  
  
  def main():
      wait_for_maintenance(maintenance_callback)
  
  
  if __name__ == '__main__':
      main()
  # [END all]
  
subject: Remove comments about possible maintenance events during live migration (#1906)