CODEWITHSUNDEEP
pythonlist
Ank
Ank

Reputation: 6260

python list concatenation

I don't quite understand whats happening here

a=[1,2,3,4]
b=[5,6,7,8]

a[len(a):] = b seems to be an equivalent of a.extend(b). when I say a[(len(a):] don't I mean a[4:<end of list a>] = b which is

`nothing in a = b which is

[]

Why am I getting [1,2,3,4,5,6,7,8] as the result?

Upvotes: 1

Views: 1319

Answers (2)

Jonathon Reinhart
Jonathon Reinhart

Reputation: 137398

The slice notation is [start : stop]. Assignment to a list slice like [start:] causes it to insert the right-hand side sequence at index start. But because there is no stop part of the slice, it will always add the entire sequence (without upper bound).

In your case, you are inserting at index 4, which doesn't exist yet, so it inserts it at the end of the original sequence.

Python Docs 3.1.4 Lists

Upvotes: 1

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 798486

Slice assignment can be used to insert one sequence within another (mutable).

>>> a = [1, 2, 3]
>>> a[1:1] = [4, 5]
>>> a
[1, 4, 5, 2, 3]

Your example is simply an instance of inserting at the end.

Upvotes: 10

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