def FindGreetingsContainerId(service, account_id):
  """Find the greetings container ID.

  Args:
    service: the Tag Manager service object.
    account_id: the ID of the Tag Manager account from which to retrieve the
      Greetings container.

  Returns:
    The dictionary that represents the greetings container if it exists, or None
    if it does not.
  """
  # Query the Tag Manager API to list all containers for the given account.
  container_wrapper = service.accounts().containers().list(
      accountId=account_id).execute()

  # Find and return the Greetings container if it exists.
  for container in container_wrapper['containers']:
    if container['name'] == 'CONTAINER NAME':
      return container['containerId']
  return None