更新时间:2023-02-26 16:16:19
没有 numpy ( ndarray.flatten
) 一种方法是使用 chain.from_iterable
这是 itertools.chain
的替代构造函数:
或者作为另一种 Pythonic 方法,您可以使用列表推导:
[j for sub in [[1,2,3],[1,2],[1,4,5,6,7]] for j in sub]
另一种非常适合短列表的函数式方法也可以是 Python2 中的 reduce
和 Python3 中的 functools.reduce
(不要将它用于长列表):
In [4]: from functools import reduce # Python3在 [5] 中:reduce(lambda x,y :x+y ,[[1,2,3],[1,2],[1,4,5,6,7]])输出[5]:[1, 2, 3, 1, 2, 1, 4, 5, 6, 7]
为了让它稍微快一点,您可以使用内置的 operator.add
代替 lambda
:
In [6]: from operator import add在 [7] 中:reduce(add ,[[1,2,3],[1,2],[1,4,5,6,7]])输出[7]: [1, 2, 3, 1, 2, 1, 4, 5, 6, 7]在 [8]: %timeit reduce(lambda x,y :x+y ,[[1,2,3],[1,2],[1,4,5,6,7]])每个循环 789 ns ± 7.3 ns(7 次运行的平均值 ± 标准偏差,每次 1000000 次循环)在 [9]: %timeit reduce(add ,[[1,2,3],[1,2],[1,4,5,6,7]])每个循环 635 ns ± 4.38 ns(7 次运行的平均值 ± 标准偏差,每次 1000000 次循环)
基准:
:~$ python -m timeit "from itertools import chain;chain.from_iterable([[1,2,3],[1,2],[1,4,5,6,7]])"1000000 个循环,***的 3 个:每个循环 1.58 微秒:~$ python -m timeit "reduce(lambda x,y :x+y ,[[1,2,3],[1,2],[1,4,5,6,7]])";1000000 个循环,***的 3 个:每个循环 0.791 微秒:~$ python -m timeit "[j for i in [[1,2,3],[1,2],[1,4,5,6,7]] for j in i]";1000000 个循环,***的 3 个:每个循环 0.784 微秒
使用 sum
的@Will 答案的基准(它对短名单来说很快,但对长名单来说不是很快):
:~$ python -m timeit "sum([[1,2,3],[4,5,6],[7,8,9]], [])";1000000 个循环,***的 3 个:每个循环 0.575 微秒:~$ python -m timeit "sum([range(100),range(100)], [])";100000 个循环,***的 3 个:每个循环 2.27 微秒:~$ python -m timeit "reduce(lambda x,y :x+y ,[range(100),range(100)])";100000 个循环,***的 3 个:每个循环 2.1 usec
I have a list looks like this:
[[1,2,3],[1,2],[1,4,5,6,7]]
and I want to flatten it into [1,2,3,1,2,1,4,5,6,7]
is there a light weight function to do this without using numpy?
Without numpy ( ndarray.flatten
) one way would be using chain.from_iterable
which is an alternate constructor for itertools.chain
:
>>> list(chain.from_iterable([[1,2,3],[1,2],[1,4,5,6,7]]))
[1, 2, 3, 1, 2, 1, 4, 5, 6, 7]
Or as another yet Pythonic approach you can use a list comprehension :
[j for sub in [[1,2,3],[1,2],[1,4,5,6,7]] for j in sub]
Another functional approach very suitable for short lists could also be reduce
in Python2 and functools.reduce
in Python3 (don't use this for long lists):
In [4]: from functools import reduce # Python3
In [5]: reduce(lambda x,y :x+y ,[[1,2,3],[1,2],[1,4,5,6,7]])
Out[5]: [1, 2, 3, 1, 2, 1, 4, 5, 6, 7]
To make it slightly faster you can use operator.add
, which is built-in, instead of lambda
:
In [6]: from operator import add
In [7]: reduce(add ,[[1,2,3],[1,2],[1,4,5,6,7]])
Out[7]: [1, 2, 3, 1, 2, 1, 4, 5, 6, 7]
In [8]: %timeit reduce(lambda x,y :x+y ,[[1,2,3],[1,2],[1,4,5,6,7]])
789 ns ± 7.3 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
In [9]: %timeit reduce(add ,[[1,2,3],[1,2],[1,4,5,6,7]])
635 ns ± 4.38 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
benchmark:
:~$ python -m timeit "from itertools import chain;chain.from_iterable([[1,2,3],[1,2],[1,4,5,6,7]])"
1000000 loops, best of 3: 1.58 usec per loop
:~$ python -m timeit "reduce(lambda x,y :x+y ,[[1,2,3],[1,2],[1,4,5,6,7]])"
1000000 loops, best of 3: 0.791 usec per loop
:~$ python -m timeit "[j for i in [[1,2,3],[1,2],[1,4,5,6,7]] for j in i]"
1000000 loops, best of 3: 0.784 usec per loop
A benchmark on @Will's answer that used sum
(its fast for short list but not for long list) :
:~$ python -m timeit "sum([[1,2,3],[4,5,6],[7,8,9]], [])"
1000000 loops, best of 3: 0.575 usec per loop
:~$ python -m timeit "sum([range(100),range(100)], [])"
100000 loops, best of 3: 2.27 usec per loop
:~$ python -m timeit "reduce(lambda x,y :x+y ,[range(100),range(100)])"
100000 loops, best of 3: 2.1 usec per loop