Python NameError: name 'reduce' is not defined

今天在搜用Python求阶乘的时候, 搜出来的最简单的是用reduce这个built-in function, 但是我在用reduce的时候, 却报NameError: name 'reduce'
is not defined. 于是又搜了一下,发现在python 3.0.0.0以后, reduce已经不在built-in function里了, 要用它就得from functools import
reduce.

详见The fate of reduce() in Python 3000

 

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.

 

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