list.extend和list comprehension

Question

当我需要在列表中添加几个相同的项目时,我使用list.extend:

a = ['a', 'b', 'c']
a.extend(['d']*3)

结果

['a', 'b', 'c', 'd', 'd', 'd']

但是,如何与列表理解类似？

a = [['a',2], ['b',2], ['c',1]]
[[x[0]]*x[1] for x in a]

结果

[['a', 'a'], ['b', 'b'], ['c']]

但我需要这个

['a', 'a', 'b', 'b', 'c']

有任何想法吗？

Answer 1

堆叠式LC.

[y for x in a for y in [x[0]] * x[1]]

Answer 2

>>> a = [['a',2], ['b',2], ['c',1]]
>>> [i for i, n in a for k in range(n)]
['a', 'a', 'b', 'b', 'c']

Answer 3

一个itertools方法：

import itertools

def flatten(it):
    return itertools.chain.from_iterable(it)

pairs = [['a',2], ['b',2], ['c',1]]
flatten(itertools.repeat(item, times) for (item, times) in pairs)
# ['a', 'a', 'b', 'b', 'c']

Answer 4

如果您更喜欢扩展而不是列表理解：

a = []
for x, y in l:
    a.extend([x]*y)