Generate a list with new items containing more than one item


I have an array a. I want to create new array with doubled size where items is x*2 and x*3.

For example: a = [1,10,100] result must be b = [2,3,20,30,200,300]

I know this (ugly and very slow) way: b = sum([[x*2,x*3] for x in a], [])

There is other way (truly I want shortest way :)?

This can be done using a list comprehension with nested loops

In [4]: [y for x in a for y in (x * 2, x * 3)]
Out[4]: [2, 3, 20, 30, 200, 300]

Seems to outperform all answers, but loses to the numpy solution when a is large.