Pythonic way to split a list into first and rest?

Question

I think in Python 3 I'll be able to do:

first, *rest = l

which is exactly what I want, but I'm using 2.6. For now I'm doing:

first = l[0]
rest = l[1:]

This is fine, but I was just wondering if there's something more elegant.

I think the above is fine. You could always do `first, rest = l[0], l[1:]`. — Katriel, Aug 18 '10 at 15:52
Indeed, you can't improve on the above -- http://www.python.org/dev/peps/pep-3132/. — Katriel, Aug 18 '10 at 15:56
Related: https://stackoverflow.com/questions/10532473/python-head-and-tail-in-one-line — Denilson Sá Maia, Mar 01 '16 at 20:37

score 54 · Accepted Answer · edited Aug 18 '10 at 15:56

54

first, rest = l[0], l[1:]

Basically the same, except that it's a oneliner. Tuple assigment rocks.

This is a bit longer and less obvious, but generalized for all iterables (instead of being restricted to sliceables):

i = iter(l)
first = next(i) # i.next() in older versions
rest = list(i)

edited Aug 18 '10 at 15:56

NullUserException

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answered Aug 18 '10 at 15:53

5

Better is: `first, last = l[:1], l[1:]` in case `l` is empty (and it's prettier ;). – Don O'Donnell Aug 18 '10 at 21:26
9

Should mention these are not semantically the same: `l[:1]` is a list, `l[0] ` is an element of the list. – Don O'Donnell Aug 18 '10 at 22:11

score 21 · Answer 2 · answered Aug 18 '10 at 16:06

21

You can do

first = l.pop(0)

and then l will be the rest. It modifies your original list, though, so maybe it’s not what you want.

answered Aug 18 '10 at 16:06

Olivier 'Ölbaum' Scherler

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This won't work for empty lists: ```x = []; x.pop(0) Traceback (most recent call last): File "", line 1, in IndexError: pop from empty list ``` – avyfain Jul 19 '17 at 01:01
@avyfain: neither would work the Python3 example of the OP. `first, *rest = []` raise a ValueError. – kriss Apr 04 '19 at 12:22

score 1 · Answer 3 · edited Oct 19 '21 at 13:42

1

If l is string typeI would suggest:

first, remainder = l.split(None, maxsplit=1)

edited Oct 19 '21 at 13:42

Saeed

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answered Jul 08 '16 at 14:12

FGHP

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`list` does not have a `.split()` method. – Uyghur Lives Matter Jul 08 '16 at 19:08
ooops ! my mistake, though it was for a string – FGHP Jul 10 '16 at 11:23
3

Still +1 for while it is wrong in this case, it was a good thing, because I wanted to split string and assign head + rest of result to different variables. – Gnudiff Jul 04 '19 at 09:06
Note this throws an error if the split only returns a list of 1 item – Peter Gibson Jul 26 '21 at 01:24

score 0 · Answer 4 · answered Apr 04 '19 at 12:29

0

Yet another one, working with python 2.7. Just use an intermediate function. Logical as the new behavior mimics what happened for functions parameters passing.

li = [1, 2, 3]
first, rest = (lambda x, *y: (x, y))(*li)

answered Apr 04 '19 at 12:29

kriss

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Pythonic way to split a list into first and rest?

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