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?
3 Answers
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...
5 Comments
Tyler Cowan
is there a differencde between extend and []+[]
Adam Smith
@TylerCowan The former modifies one list and returns
None, the latter returns a new list containing all the elements. Otherwise no.
Julio Marins
the last example only works on python3
Mincho
the [] + [] takes more memory than .extend() because its a 2-step process
Adam Smith
@Mincho that's probably true, but maybe not for the reason you're thinking. Certainly not because it's a 2-step process. after
c = a + b you have three lists, a, b, and c. Each one has to be held in memory until it falls out of scope (unless you del a; del b). After a.extend(b) you have only two lists -- a and b. That said: the biggest difference is still that list.extend mutates the existing list while list.__add__ produces a new one.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]
Comments
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)
3 Comments
Adam Smith
bah, I missed one ;) May be worth pointing out specifically that
list.extend returns None, so not to use b = b.extend(a). That's a common mistake from beginner Python users.Adam Smith
Good point, and one that was lacking from my own answer. Thanks!
b = [10] + a