Can I extend list in Python with prepend elements instead of append?

Question

I can perform

a = [1,2,3]
b = [4,5,6]
a.extend(b)
# a is now [1,2,3,4,5,6]

Is there way to perform an action for extending list and adding new items to the beginning of the list?

Like this

a = [1,2,3]
b = [4,5,6]
a.someaction(b)
# a is now [4,5,6,1,2,3]

I use version 2.7.5, if it is important.

@iCodez: sometimes `a` is called `very_important` and `b` is called `aux`. You may want to keep the former and forget about the latter. — Peque, Jul 31 '14 at 14:14

Martijn Pieters · Accepted Answer · 2016-05-29T18:03:44.880

99

You can assign to a slice:

a[:0] = b

Demo:

>>> a = [1,2,3]
>>> b = [4,5,6]
>>> a[:0] = b
>>> a
[4, 5, 6, 1, 2, 3]

Essentially, list.extend() is an assignment to the list[len(list):] slice.

You can 'insert' another list at any position, just address the empty slice at that location:

>>> a = [1,2,3]
>>> b = [4,5,6]
>>> a[1:1] = b
>>> a
[1, 4, 5, 6, 2, 3]

edited May 29 '16 at 18:03

answered Nov 01 '13 at 21:58

Martijn Pieters

1,048,767
296
4,058
3,343

Sadly, a[-1:] doesn't work (for obvious reasons). It would be so nice to have symmetry, but alas… – Jürgen A. Erhard Aug 05 '21 at 11:07
@JürgenA.Erhard: There absolutely is symmetry, but `-1` is not the end of the list, it is the last element. `a[-1:]` is symmetrical with `a[:1]` at the start of the list, both contain 1 element. This is why I explicitly mention `len(list)` in my answer. – Martijn Pieters Aug 05 '21 at 13:35

score 17 · Answer 2 · answered Nov 01 '13 at 22:00

17

This is what you need ;-)

a = b + a

answered Nov 01 '13 at 22:00

Artur

7,038
2
25
39

u'll end up adding the corresponding array elements. – magarwal Jul 22 '15 at 03:04
1

@magarwal Only if you are using numpy. – Mad Physicist Oct 27 '15 at 14:33
1

not if you need to modify `a`, try this: `>>> a is b+a` (`b+a` is neither `b` nor `a`, but a completely new object) ;) – Tcll Mar 25 '18 at 03:29

score 4 · Answer 3 · answered Nov 01 '13 at 22:01

4

You could use collections.deque:

import collections
a = collections.deque([1, 2, 3])
b = [4, 5, 6]
a.extendleft(b[::-1])

answered Nov 01 '13 at 22:01

TerryA

58,805
11
114
143

This is the best way if you don't need to be able to access arbitrary elements. insert and adding lists tends to yield quadratic algorithms, where linear is possible. – dstromberg Nov 01 '13 at 22:52

score 3 · Answer 4 · answered Nov 01 '13 at 22:59

If you need fast operations and you need to be able to access arbitrary elements, try a treap or red-black tree.

>>> import treap as treap_mod
>>> treap = treap_mod.treap()
>>> for i in range(100000):
...    treap[i] = i
...
>>> treap[treap.find_min() - 1] = -1
>>> treap[100]
100

Most operations on treaps and red-black trees can be done in O(log(n)). Treaps are purportedly faster on average, but red-black trees give a lower variance in operation times.

Can I extend list in Python with prepend elements instead of append?

4 Answers4

Related