如何从列表列表中制作平面列表 [英] How to make a flat list out of a list of lists

问题描述

有没有捷径可以从 Python 中的列表列表中创建一个简单的列表?

Is there a shortcut to make a simple list out of a list of lists in Python?

我可以在 for 循环中完成，但是有没有一些很酷的单行"?

I can do it in a for loop, but is there some cool "one-liner"?

我用 functools.reduce() 试过了:

from functools import reduce
l = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
reduce(lambda x, y: x.extend(y), l)

但我收到此错误:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <lambda>
AttributeError: 'NoneType' object has no attribute 'extend'

推荐答案

给定一个列表 t,

flat_list = [item for sublist in t for item in sublist]

意思是:

flat_list = []
for sublist in t:
    for item in sublist:
        flat_list.append(item)

比目前发布的快捷方式更快.(t 是要展平的列表.)

is faster than the shortcuts posted so far. (t is the list to flatten.)

这里是对应的函数:

def flatten(t):
    return [item for sublist in t for item in sublist]

作为证据，您可以使用标准库中的 timeit 模块:

As evidence, you can use the timeit module in the standard library:

$ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' '[item for sublist in t for item in sublist]'
10000 loops, best of 3: 143 usec per loop
$ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'sum(t, [])'
1000 loops, best of 3: 969 usec per loop
$ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'reduce(lambda x,y: x+y,t)'
1000 loops, best of 3: 1.1 msec per loop

说明:基于+的快捷键(包括sum中的隐含使用)，必然是O(T**2) 当有 T 个子列表时——随着中间结果列表越来越长，每一步都会分配一个新的中间结果列表对象，并且必须复制前一个中间结果中的所有项(以及一些新的项)最后补充).因此，为简单起见且不失一般性，假设每个子列表都有 k 个项目:前 k 个项目来回复制 T-1 次，后 k 个项目 T-2 次，依此类推；总拷贝数是x从1到T排除x的总和的k倍，即k * (T**2)/2.

Explanation: the shortcuts based on + (including the implied use in sum) are, of necessity, O(T**2) when there are T sublists -- as the intermediate result list keeps getting longer, at each step a new intermediate result list object gets allocated, and all the items in the previous intermediate result must be copied over (as well as a few new ones added at the end). So, for simplicity and without actual loss of generality, say you have T sublists of k items each: the first k items are copied back and forth T-1 times, the second k items T-2 times, and so on; total number of copies is k times the sum of x for x from 1 to T excluded, i.e., k * (T**2)/2.

列表推导式只生成一个列表，一次，并将每个项目(从其原始居住地到结果列表)复制一次.

The list comprehension just generates one list, once, and copies each item over (from its original place of residence to the result list) also exactly once.

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