Concatenation of many lists in Python [duplicate]

Suppose I have a function like this:

def getNeighbors(vertex)

which returns a list of vertices that are neighbors of the given vertex. Now I want to create a list with all the neighbors of the neighbors. I do that like this:

listOfNeighborsNeighbors = []
for neighborVertex in getNeighbors(vertex):
    listOfNeighborsNeighbors.append(getNeighbors(neighborsVertex))

Is there a more pythonic way to do that?

As usual, the itertools module contains a solution:

>>> l1=[1, 2, 3]

>>> l2=[4, 5, 6]

>>> l3=[7, 8, 9]

>>> import itertools

>>> list(itertools.chain(l1, l2, l3))
[1, 2, 3, 4, 5, 6, 7, 8, 9]

[x for n in getNeighbors(vertex) for x in getNeighbors(n)]

or

sum(getNeighbors(n) for n in getNeighbors(vertex), [])

Appending lists can be done with + and sum():

>>> c = [[1, 2], [3, 4]]
>>> sum(c, [])
[1, 2, 3, 4]