回复内容：
reduce翻译成规约，意思其他答案里面已经讲了。
实际上函数名不应取reduce，应该叫fold，而且要区分foldl和foldr左右两个版本。并行计算里面的那个reduce有个隐含前提，是reduce over monoid，所以结果才与reduce方向无关，可以做有效的并行化。
归约
这是一个函数式编程里面，高阶函数里面很基础的概念。不过我最早听到这个词是在并行计算里面。
函数式编程里面有时也称作 fold ，也就是折叠，这个更好理解吧。详情可参见wikipedia，里面提到了 In functional programming , fold – also known variously as reduce , accumulate , aggregate , compress , or inject
你找一个你觉得好理解的词就OK了。
这就是MapReduce中的reduce，也叫fold。
wikipedia链接：Fold (higher-order function)
SICP中用scheme实现的叫accumulate。
C++中是std::accumulate。
wiki中 Folds in various languages 部分讲了各种语言中的这个函数的叫法。
reduce在函数式语言里就是要减少的意思，要合并结果，多项变成一项。至于怎么合并，就看reduce的函数是什么。
range得到一个collection（或sequence），里面有多项。reduce函数是add，结果是一项。
当然reduce后也可以还是一个collection，那是因为把collection整体看做一项结果。
别纠结了，这个函数如此难懂，完全不符合python的风格，以致于在python3中被删除了。
题主还是趁早改邪归正，抛弃python2，投入python3的阵营吧！
reduce 在这里应该翻译成“归纳”。
verb (used with object), brought, bringing.
1.
to carry, convey, conduct, or cause (someone or something) to comewith, to, or toward the speaker:
Bring the suitcase to my house. He brought his brother to my office.
2.
to cause to come to or toward oneself; attract:
Her scream brought the police. He brought honor to his family by hisheroism.
3.
to cause to occur or exist:
The medication brought instant relief.
4.
to cause to come into a particular position, state, or effect:
to bring the car to a stop.
5.
to cause to appear or occur in the mind; evoke or recall:
The letter brought her memories of youth.
6.
to persuade, convince, compel, or induce:
She couldn't bring herself to sell the painting.
7.
to sell for; fetch:
These lamps will bring a good price.
Verb phrases
9.
bring about, to accomplish; cause:
Land reform brought about a great change in the lives of the commonpeople.
10.
bring around /round, to convince of a belief or opinion; persuade:
I think we can bring him around to agreeing with the plan.
to restore to consciousness, as after a faint. to bring as a visitor:
They brought around a new employee this morning.
11.
bring down, to injure, capture, or kill:
He brought down several ducks on his last hunting trip.
to lessen; reduce:
I won't buy that lamp unless they bring down the price.
Slang. to cause to be in low spirits; depress:
The bad news brought him down.
12.
bring forth, to give birth to; deliver; bear:
to bring forth a son.
to give rise to; introduce:
to bring forth a proposal for reducing costs.
13.
bring forward, to bring to view; show. to present for consideration; adduce:
to bring forward an opinion.
14.
bring in, to yield, as profits or income:
My part-time job doesn't bring in much, but I enjoy it.
to present officially; submit:
The jury brought in its verdict.
to cause to operate or yield:
They brought in a gusher on his property.
to present for consideration, approval, etc.; introduce:
She brought in six new members last month.
15.
bring off, to accomplish, carry out, or achieve (something):
He brought off his speech with ease.
16.
bring on, to cause to happen or exist; bring about:
This incident will surely bring on a crisis.
to introduce; cause to appear:
Bring on the clowns.
17.
bring out, to expose; reveal. to make noticeable or conspicuous in a contrast. to publish, as a book or play. to introduce officially into society:
to bring out a debutante.
18.
bring to, to bring back to consciousness; revive. Nautical. to head (a vessel) close to or into the wind so as to halt. 19.
bring up, to care for during childhood; rear. to introduce or mention for attention, discussion, action, orconsideration. to vomit. to stop or cause to stop quickly:
to bring up a car at the curb.
Nautical. (of a vessel) to cause to halt, as by lowering an anchor orrunning aground; fetch up.
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reduce 很多项成一项啊。这个很难理解么？
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