Python flatten list of lists into list

For scenarios where a function outputs a list, and that function is in a for loop or asyncio event loop, the final output will be a list of lists, like:

x = [[1,2,3], [4, 5, 6]]

This may be inconvenient for applications where a flattened list is required. The simplest and fastest way to flatten a list of lists in like:

import itertools

x = [[1,2,3], [4, 5, 6]]

x_flat = list(itertools.chain(*x))

which results in

[1, 2, 3, 4, 5, 6]