Extend list with another list in specific index?

Question

In python we can add lists to each other with the extend() method but it adds the second list at the end of the first list.

lst1 = [1, 4, 5]
lst2 = [2, 3]

lst1.extend(lst2)

Output:
[1, 4, 5, 2, 3]

How would I add the second list to be apart of the 1st element? Such that the result is this;

[1, 2, 3, 4, 5 ]

I’ve tried using lst1.insert(1, *lst2) and got an error;

TypeError: insert expected 2 arguments, got 3

Asked By: Yussef Raouf Abdelmisih

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Source

Answer 1

You can solve your problem in two steps:

Insert the list into the other list
Flatten the result

Code:

from collections.abc import Iterable

# https://stackoverflow.com/questions/2158395/flatten-an-irregular-arbitrarily-nested-list-of-lists
def flatten(xs):
    for x in xs:
        if isinstance(x, Iterable) and not isinstance(x, (str, bytes)):
            yield from flatten(x)
        else:
            yield x

xs = [1,4,5]
ys = [2,3]
xs.insert(1, ys)
print("intermediate result", xs)
xs = flatten(xs)
print(xs)

Answered By: Caridorc

Answer 2

For those who don’t like reading comments:

lst1 = [1, 4, 5]
lst2 = [2, 3]

lst1[1:1] = lst2
print(lst1)

Output:

[1, 2, 3, 4, 5]

Answered By: Jamiu Shaibu

Answer 3

If your only goal is to get the list sorted correctly, then you use .extend() and .sort() afterwards.

Answered By: Mou

Answer 4

@Jamiu has the correct solution if the list are always sorted.

if lists are not in order then

lst1 = sorted(lst1+lst2)

Answered By: Hackaholic

Extend list with another list in specific index?

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