Join Lists

Put two lists together, and turn a list into a string.

Adding two lists

+ makes a new list from two:

Python

first = [1, 2]
second = [3, 4]
print(first + second)
print(first)

Output

[1, 2, 3, 4]
[1, 2]

Neither original changed.

Extending in place

extend() and += add the items to the list you already have:

Python

first = [1, 2]
first.extend([3, 4])
print(first)

other = [1]
other += [2, 3]
print(other)

Output

[1, 2, 3, 4]
[1, 2, 3]

Use + when you want a new list, extend() when you want to grow this one.

Unpacking into a new list

The star spreads a list into a new one, and takes any number of sources:

Python

first = [1, 2]
second = [3]
third = [4, 5]
print([*first, *second, *third])
print([0, *first, 99])

Output

[1, 2, 3, 4, 5]
[0, 1, 2, 99]

This is often the clearest option, because it puts everything in one expression without touching the originals.

Flattening a list of lists

Python

groups = [[1, 2], [3], [4, 5]]
flat = [item for group in groups for item in group]
print(flat)

Output

[1, 2, 3, 4, 5]

itertools.chain does the same without building the intermediate list:

Python

from itertools import chain
groups = [[1, 2], [3], [4, 5]]
print(list(chain.from_iterable(groups)))

Output

[1, 2, 3, 4, 5]

Interleaving with zip

Python

names = ["Ada", "Grace"]
years = [1815, 1906]
print(list(zip(names, years)))

Output

[('Ada', 1815), ('Grace', 1906)]

Turning a list into a string

This is str.join(), and the separator is the string you call it on:

Python

names = ["Ada", "Grace", "Katherine"]
print(", ".join(names))
print("".join(["a", "b", "c"]))

Output

Ada, Grace, Katherine
abc

Python

numbers = [1, 2, 3]
print(", ".join(str(n) for n in numbers))

Output

1, 2, 3

Repeating a list

* repeats, the same as with strings:

Python

print([0] * 5)

Output

[0, 0, 0, 0, 0]

Python

grid = [[0] * 2] * 3
grid[0][0] = 99
print(grid)

Output

[[99, 0], [99, 0], [99, 0]]

Build each row separately instead:

Python

grid = [[0] * 2 for _ in range(3)]
grid[0][0] = 99
print(grid)

Output

[[99, 0], [0, 0], [0, 0]]

Test yourself

2 questions

What does ", ".join([1, 2, 3]) do?

Show the answer

Raises TypeError — join needs every item to be a string already. Convert first with str().

Why is [[0] 2] 3 dangerous?

Show the answer

All three rows are the same list, so changing one changes all — Repeating copies the reference, not the list. Build each row separately in a comprehension.

Next chapter

List Methods

The full set of list methods, and which return a value rather than None.