How to spread a python array

Question

In JS I can do this

const a = [1,2,3,4]
const b = [10, ...a]
console.log(b) // [10,1,2,3,4]

Is there a similar way in python?

Asked By: Alex Cory

||

Source

Answer 1

As Alexander points out in the comments, list addition is concatenation.

a = [1,2,3,4]
b = [10] + a  # N.B. that this is NOT `10 + a`
# [10, 1, 2, 3, 4]

You can also use list.extend

a = [1,2,3,4]
b = [10]
b.extend(a)
# b is [10, 1, 2, 3, 4]

and newer versions of Python allow you to (ab)use the splat (*) operator.

b = [10, *a]
# [10, 1, 2, 3, 4]

Your choice may reflect a need to mutate (or not mutate) an existing list, though.

a = [1,2,3,4]
b = [10]
DONTCHANGE = b

b = b + a  # (or b += a)
# DONTCHANGE stays [10]
# b is assigned to the new list [10, 1, 2, 3, 4]

b = [*b, *a]
# same as above

b.extend(a)
# DONTCHANGE is now [10, 1, 2, 3, 4]! Uh oh!
# b is too, of course...

Answered By: Adam Smith

Answer 2

Python’s list object has the .extend function.

You can use it like this:

    a = [1, 2, 3, 4]
    b = [10]
    b.extend(a)
    print(b)

Answered By: user9232447

Answer 3

The question does not make clear what exactly you want to achieve.

To replicate that operation you can use the Python list extend method, which appends items from the list you pass as an argument:

>>> list_one = [1,2,3]
>>> list_two = [4,5,6]
>>> list_one.extend(list_two)
>>> list_one
[1, 2, 3, 4, 5, 6]

If what you need is to extend a list at a specific insertion point you can use list slicing:

>>> l = [1, 2, 3, 4, 5]
>>> l[2:2] = ['a', 'b', 'c']
>>> l
[1, 2, 'a', 'b', 'c', 3, 4, 5]

Answered By: Francisco Jiménez Cabrera

How to spread a python array

Question:

Answers: