假设你有两个函数。下面的代码示例的输出是21。
为什么
首先，x、y来自lst的第一、第二元素。在continue中，x来自函数的返回，y来自lst的元素。

1. x=1，y=2 -> one(1,2) -> 1+2+1 -> 4
2. x=4，y=3 -> two(4,3) -> 4+3+2 -> 9
3. x=9，y=4 -> one(9,4) -> 9+4+1 -> 14
4. x=14，y=5 -> two(14,5) -> 14+5+2 -> 21 -> return

from functools import reduce
from itertools import cycle

def one(x,y):
    return x+y+1
def two(x,y):
    return x+y+2

rules = [one, two]
lst = [1,2,3,4,5]

def reduce_by_rules(lst, rules):
    rs = cycle(rules)
    return reduce(lambda x, y: next(rs)(x, y), lst)
    # return two(one(two(one(1,2), 3), 4), 5)

print(reduce_by_rules(lst, rules))
# 21

cycle(rules)只是一组重复的函数，例如：

def multiply(x,y):
   return x*y
def divide(x,y):
   return x/y

rules = (multiply, divide)
lst = [range(100)]

def reduce_by_rules(lst, rules):
  rs = cycle(rules) # -> multiply, divide, multiply, divide, ...repeats forever
  # reduce applies the function to each number and return a number reduce(foo, [1,2,3,4]) => foo(foo(foo(1,2),3),4) 
  return reduce(lambda x, y: next(rs)(x, y), lst)

reduce_by_rules(lst, rules)

如果它帮助lemme定义我自己的循环函数：

def my_cyle(lst):
   while True:
      for element in lst:
           yield element

我自己的reduce：

def my_reduce(function, lst):
   res = lst[0]
   for e in lst[1:]:
       res = function(res, e)
   return res