How to spread a python array [duplicate]

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?

Solution 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...

Solution 2:

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]

Solution 3:

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)