Possible to append multiple lists at once? (Python)

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]

equivalent to above answer, but sufficiently different to be worth a mention:

my_map = {
   'foo': ['a', 1, 2],
   'bar': ['b', '2', 'c'],
   'baz': ['d', 'e', 'f'],
} 
list(itertools.chain(*my_map.values()))
['d', 'e', 'f', 'a', 1, 2, 'b', '2', 'c']

in the above expression, * is important for groking result as args to chain, this is same as prior chain(x,y,z). Also, note the result is hash-ordered.

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.

If you prefer a slightly more functional approach, you could try:

import functools as f

x = [1, 2, 3]
y = [4, 5, 6]
z = [7, 8, 9]

x = f.reduce(lambda x, y: x+y, [y, z], x)

This will enable you to concatenate any number of lists onto list x.

If you would just like to concatenate any number of lists together (i.e. not onto some base list), you can simplify to:

import functools as f
from operator import add
big_list = f.reduce(add, list_of_lists)

Take note that our BFDL has his reservations with regard to lambdas, reduce, and friends: https://www.artima.com/weblogs/viewpost.jsp?thread=98196

To complete this answer, you can read more about reduce in the documentation: https://docs.python.org/3/library/functools.html#functools.reduce

I quote: "Apply function of two arguments cumulatively to the items of sequence, from left to right, so as to reduce the sequence to a single value."

P.S. https://stackoverflow.com/a/33277438/532513 compares the performance of different approaches for list concatenation.

In one line , it can be done in following ways

x.extend(y+z)

or

x=x+y+z

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)