Concatenate lists in Python

Use a + b or [*a, *b] to create a new list. Use a.extend(b) to change an existing list, or itertools.chain(a, b) to iterate over both without first building a combined list.

Create a new list

a = [1, 2, 3]
b = [4, 5, 6]

combined = a + b
unpacked = [*a, *b]
print(combined)  # [1, 2, 3, 4, 5, 6]
print(a)         # [1, 2, 3]

List addition requires lists on both sides. Unpacking also accepts other iterables. Both create a new outer list, but they do not deep-copy its elements: nested mutable objects remain shared.

Extend a list in place

a = [1, 2, 3]
b = [4, 5, 6]
a.extend(b)
print(a)  # [1, 2, 3, 4, 5, 6]

extend() returns None, so do not write a = a.extend(b). Every reference to the original list sees the added elements. The list still needs storage for the additions; in-place does not mean no memory allocation.

Iterate without materializing the combination

from itertools import chain

a = [1, 2, 3]
b = [4, 5, 6]
for value in chain(a, b):
    print(value)

For many input lists, use chain.from_iterable(lists). It produces an iterator; wrapping it in list() allocates the combined list after all.

For example, [value for group in lists for value in group] flattens one level. Repeatedly using + in a growing loop copies intermediate lists; extend() or chain usually expresses that job better.

FAQ

How is append() different? a.append(b) adds b as one element, creating a nested list if b is a list. extend() adds each element from the iterable.

Does + change either input? No. It creates a new list. The examples above reset the inputs between methods so the in-place example does not accidentally duplicate later results.

References

• Python sequence and list operations
• Python itertools.chain