Repeat each item in a list a number of times specified in another list

Question

I have two lists, x and y:

>>> x = [2, 3, 4]
>>> y = [1, 2, 3]

I want to use these to create a new list. The new list will have each element in x repeated the number of times specified by the corresponding element in y. Hence, the desired output is

>>> new_list
[2, 3, 3, 4, 4, 4]

The order of the elements in new_list doesn't matter to me. It's also not crucial that it be a list -- any sequence type is fine.

What is the fastest, most efficient, most Pythonic way to achieve this?

Answer 1

numpy's repeat function gets the job done:

>>> import numpy as np
>>> x = [2, 3, 4]
>>> y = [1, 2, 3]
>>> np.repeat(x, y)
array([2, 3, 3, 4, 4, 4])