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How to concatenate two Lists in Python

Learn different ways to concatenate two lists or other iterables in Python.


This article shows different ways to concatenate two lists or other iterables in Python.

Use a + b

The simplest way is by just using the + operator to combine two lists:

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

c = a + b
# [1, 2, 3, 4]

Use [*a, *b]

Another alternative has been introduced in Python 3.5 via the acceptance of PEP 448.

This PEP is titled Additional Unpacking Generalizations and is a more general way to unpack and combine items.

While the + operator only works with two lists, this unpacking technique can be used for other iterables (e.g. tuples or sets), too.

c = [*a, *b]
# [1, 2, 3, 4]
a = [1, 2]
b = (3, 4)

# c = a + b 
# TypeError: can only concatenate list (not "tuple") to list

c = [*a, *b]
# [1, 2, 3, 4]

Careful: Only creates a shallow copy!

Be careful! Both mentioned methods above create only a shallow copy!

This means the copy is one level deep. Modifying on level 1 does not affect the other list. But with nested objects, modifying on level 2 or deeper does affect the other!

In this example we have a nested list. After creating a new list c, we modify an inner item of a. Notice that c now has the same modification, too!

To learn more about this, visit my blog post about shallow vs deep copying.

# nested lists
a = [[1, 2], [3, 4]]
b = [[5, 6], [7, 8]]

c = a + b
print(c)
# [[1, 2], [3, 4], [5, 6], [7, 8]]

a[0][0] = 99
print(c)
# [[99, 2], [3, 4], [5, 6], [7, 8]]

In-place updates with list.extend(iterable)

To update an existing list in-place, and add the items of another list, you can use list.extend(iterable). This also works with any type of iterable.

a = [1, 2]
b = [3, 4]  # also works with other iterables

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

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