python 3.0以后, reduce已经不在built-in function里了, 要用它就得from functools import reduce.
reduce的用法
reduce(function, sequence[, initial]) -> value
Apply a function of two arguments cumulatively to the items of a sequence,
from left to right, so as to reduce the sequence to a single value.
For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates
((((1+2)+3)+4)+5). If initial is present, it is placed before the items
of the sequence in the calculation, and serves as a default when the
sequence is empty.
意思就是对sequence连续使用function, 如果不给出initial, 则第一次调用传递sequence的两个元素, 以后把前一次调用的结果和sequence的下一个元素传递给function. 如果给出initial, 则第一次传递initial和sequence的第一个元素给function
*functool标准库还有很多功能，可以参考网上的资料
