code: def max(self, key=None):
          
          if key is None:
              return self.reduce(max)
          return self.reduce(lambda a, b: max(a, b, key=key))
docs: Find the maximum item in this RDD.
  
          :param key: A function used to generate key for comparing
  
          >>> rdd = sc.parallelize([1.0, 5.0, 43.0, 10.0])
          >>> rdd.max()
          43.0
          >>> rdd.max(key=str)
          5.0