python 3.0以後, reduce已經不在built-in function裡了, 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.

意思就是對sequence連續使用function, 如果不給出initial, 則第一次呼叫傳遞sequence的兩個元素, 以後把前一次呼叫的結果和sequence的下一個元素傳遞給function. 如果給出initial, 則第一次傳遞initial和sequence的第一個元素給function.

from functools import reduce  
reduce(lambda x,y: x+y, [1, 2, 3])  
輸出 6  
reduce(lambda x, y: x+y, [1,2,3], 9)  
輸出 15  
reduce(lambda x,y: x+y, [1, 2, 3], 7)
 
  
輸出 13  

＊functool標準庫還有很多功能，可以參考網上的資料