Possible to append multiple lists at once? (Python)

x.extend(y+z)

should do what you want

or

x += y+z

or even

x = x+y+z

Extending my comment

In [1]: x = [1, 2, 3]
In [2]: y = [4, 5, 6]
In [3]: z = [7, 8, 9]
In [4]: from itertools import chain
In [5]: print list(chain(x,y,z))
[1, 2, 3, 4, 5, 6, 7, 8, 9]

You can use sum function with start value (empty list) indicated:

a = sum([x, y, z], [])

This is especially more suitable if you want to append an arbitrary number of lists.

To exactly replicate the effect of append, try the following function, simple and effective:

a=[]
def concats (lists):
    for i in lists:
        a==a.append(i)


concats ([x,y,z])
print(a)