Given two lists:
x = [1,2,3]
y = [4,5,6]
What is the syntax to:
- Insert
xintoysuch thatynow looks like[1, 2, 3, [4, 5, 6]]? - Insert all the items of
xintoysuch thatynow looks like[1, 2, 3, 4, 5, 6]?
6 Answers
Do you mean append?
>>> x = [1,2,3]
>>> y = [4,5,6]
>>> x.append(y)
>>> x
[1, 2, 3, [4, 5, 6]]
Or merge?
>>> x = [1,2,3]
>>> y = [4,5,6]
>>> x + y
[1, 2, 3, 4, 5, 6]
>>> x.extend(y)
>>> x
[1, 2, 3, 4, 5, 6]
2 Comments
Daniel Möller
Is that in place or yields a new instance?
wim
x.extend(y) is in place, x+y is returning new list. And x += y, which was not mentioned here, is similar to the extend.The question does not make clear what exactly you want to achieve.
List has the append method, which appends its argument to the list:
>>> list_one = [1,2,3]
>>> list_two = [4,5,6]
>>> list_one.append(list_two)
>>> list_one
[1, 2, 3, [4, 5, 6]]
There's also the 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]
And of course, there's the insert method which acts similarly to append but allows you to specify the insertion point:
>>> list_one.insert(2, list_two)
>>> list_one
[1, 2, [4, 5, 6], 3, 4, 5, 6]
To extend a list at a specific insertion point you can use list slicing (thanks, @florisla):
>>> l = [1, 2, 3, 4, 5]
>>> l[2:2] = ['a', 'b', 'c']
>>> l
[1, 2, 'a', 'b', 'c', 3, 4, 5]
List slicing is quite flexible as it allows to replace a range of entries in a list with a range of entries from another list:
>>> l = [1, 2, 3, 4, 5]
>>> l[2:4] = ['a', 'b', 'c'][1:3]
>>> l
[1, 2, 'b', 'c', 5]
4 Comments
florisla
If you want to 'extend' to a specific insertion point, you can use list slicing syntax (see stackoverflow.com/a/7376026/1075152)
weaver
@florisla's comment should be the accepted answer. It's the only way to insert a list into another list in place at an arbitrary location (not just at the end).
florisla
@weaver While it's the only solution to do that (extend at a specific index), that was not the original question.
weaver
@florisla That's pedantry.
foo = [1, 2, 3]
bar = [4, 5, 6]
foo.append(bar) --> [1, 2, 3, [4, 5, 6]]
foo.extend(bar) --> [1, 2, 3, 4, 5, 6]
2 Comments
DragonKnight
how do you insert bar after 2 and keep it flatten?
[1,2,4,5,6,3]
cmm
I've found that this: foo[2:2]=bar generates a flattened list. I learned that because it was not what I wanted.
You can also just do...
x += y
2 Comments
HTNW
This should be a comment on the accepted answer, because that one mentions
x + y and x += y is just the same thing but in place.
user3707850
I do not have the reputation to do that, but if someone else can!
If you want to add the elements in a list (list2) to the end of other list (list), then you can use the list extend method
list = [1, 2, 3]
list2 = [4, 5, 6]
list.extend(list2)
print list
[1, 2, 3, 4, 5, 6]
Or if you want to concatenate two list then you can use + sign
list3 = list + list2
print list3
[1, 2, 3, 4, 5, 6]
Comments
If we just do x.append(y), y gets referenced into x such that any changes made to y will affect appended x as well. So if we need to insert only elements, we should do following:
x = [1,2,3]
y = [4,5,6]
x.append(y[:])
listA.insert(pos, x) for x in listB[::-1]?