問(wèn)題描述

從這篇文章我了解到您可以使用 sum() 連接元組:

From this post I learned that you can concatenate tuples with sum():

>>> tuples = (('hello',), ('these', 'are'), ('my', 'tuples!'))
>>> sum(tuples, ())
('hello', 'these', 'are', 'my', 'tuples!')

這看起來(lái)很不錯(cuò).但為什么這行得通?而且，這是最優(yōu)的，還是 itertools 中的某些東西比這個(gè)結(jié)構(gòu)更可取?

Which looks pretty nice. But why does this work? And, is this optimal, or is there something from itertools that would be preferable to this construct?

推薦答案

python中的加法運(yùn)算符連接元組:

the addition operator concatenates tuples in python:

('a', 'b')+('c', 'd')
Out[34]: ('a', 'b', 'c', 'd')

來(lái)自sum的文檔字符串:

返回一個(gè)開(kāi)始"值(默認(rèn)值:0)加上一個(gè)可迭代的數(shù)字

Return the sum of a 'start' value (default: 0) plus an iterable of numbers

這意味著 sum 不是從可迭代的第一個(gè)元素開(kāi)始，而是從通過(guò) start= 參數(shù)傳遞的初始值開(kāi)始.

It means sum doesn't start with the first element of your iterable, but rather with an initial value that is passed through start= argument.

默認(rèn)情況下 sum 與數(shù)字一起使用，因此默認(rèn)起始值??為 0.因此，對(duì)一個(gè)可迭代的元組求和需要從一個(gè)空元組開(kāi)始.() 是一個(gè)空元組:

By default sum is used with numeric thus the default start value is 0. So summing an iterable of tuples requires to start with an empty tuple. () is an empty tuple:

type(())
Out[36]: tuple

因此工作串聯(lián).

根據(jù)性能，這里是一個(gè)比較:

As per performance, here is a comparison:

%timeit sum(tuples, ())
The slowest run took 9.40 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 285 ns per loop


%timeit tuple(it.chain.from_iterable(tuples))
The slowest run took 5.00 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 625 ns per loop

現(xiàn)在 t2 的大小為 10000:

Now with t2 of a size 10000:

%timeit sum(t2, ())
10 loops, best of 3: 188 ms per loop

%timeit tuple(it.chain.from_iterable(t2))
1000 loops, best of 3: 526 μs per loop

因此，如果您的元組列表很小，您不必費(fèi)心.如果它是中等大小或更大的，你應(yīng)該使用 itertools.

So if your list of tuples is small, you don't bother. If it's medium size or larger, you should use itertools.

這篇關(guān)于使用 sum() 連接元組的文章就介紹到這了，希望我們推薦的答案對(duì)大家有所幫助，也希望大家多多支持html5模板網(wǎng)！