This is an excerpt from the song, "Always Look on the Bright Side of Life" from Monty Python's Life of Brian

" Always look on the bright side of life...
   If life seems jolly rotten,
   There's something you've forgotton!
    That's to laugh and smile and sing,

   When you're feeling in the dumps,
   Don't be silly chumps,
   Just purse your lips and whistle-- that's the thing!
   And... always look on the bright side of life....