new_contents: import time
  
  
  class TimeOnce:
      """Time a sequence of code, allowing access to the time difference in seconds.
  
      Example without exception:
  
          elapsed = TimeOnce()
          with elapsed:
              print('sleeping ...')
              time.sleep(3)
          print("elapsed", elapsed)
  
      Example with exception:
  
          elapsed = TimeOnce()
          try:
              with elapsed:
                  print('sleeping ...')
                  time.sleep(3)
                  raise ValueError('foo')
              print("elapsed inner", elapsed)
          finally:
              print("elapsed outer", elapsed)
      """
      def __init__(self):
          self.t0 = None  # An invalid value.
  
      def __enter__(self):
          self.t0 = time.time()
  
      def __exit__(self, type, value, traceback):
          self.dt = time.time() - self.t0
  
      def get_elapsed(self):
          return self.dt
  
      def __str__(self):
          return str(self.dt)
  
subject: Implement TimeOnce class