from operator import add
sc.parallelize([1, 2, 3, 4, 5]).reduce(add)
# 15
sc.parallelize((2 for _ in range(10))).map(lambda x: 1).cache().reduce(add)
# 10
sc.parallelize([]).reduce(add)
"""
Traceback (most recent call last):
    ...
ValueError: Can not reduce() empty RDD
"""