How to pass a list by reference?

Question

I'm trying to implement a function 'add' that combines the list L1 with L2 into L3:

def add(L1,L2,L3):
    L3 = L1 + L2

L3 = []
add([1],[0],L3)
print L3

The code above produces an empty list as a result instead of [1,0] - This means that L3 wasn't passed by reference.

How to pass L3 by reference?

Answer 1

Lists are already passed by reference, in that all Python names are references, and list objects are mutable. Use slice assignment instead of normal assignment.

def add(L1, L2, L3):
    L3[:] = L1 + L2

However, this isn't a good way to write a function. You should simply return the combined list.

def add(L1, L2):
    return L1 + L2

L3 = add(L1, L2)

Answer 2

You can achieve this with:

L3[:] = L1 + L2

Test Code:

def add(L1, L2, L3):
    L3[:] = L1 + L2

L3 = []
add([1], [0], L3)
print(L3)

Results:

[1, 0]