python accessing the second to the last element in a list

Question

I have created a dictionary which I have assigned the first element of a list as a key. I would like to append the rest of the elements as values to the dictionary:

file_a contains tab delimited fields.

a = {}
for line in file_a.readlines():
    split_line = line.strip().split('\t')
    a[split_line[0]] = []
    a[split_line[0]].append(split_line[::-1]) # append from second to last element to the list

The ::-1 appends all the elements. I need to append all elements except the first one (as that is used as a key). Any help will be appreciated.

e.g. if fields are: X\t1\t2\t3 I would like the hash to be:

'X': ['1', '2', '3']

`split_line[1:][::-1]` – Burhan Khalid Jun 11 '14 at 10:31 — Burhan Khalid, Jun 11 '14 at 10:31

Saimadhav Heblikar · Accepted Answer · 2014-06-11T10:50:51.860

5

The Python slice syntax is alist[start:end:step]. So, with your slice ::-1, you are just reversing the list. If you want the second element to the last, the correct slice would be alist[1:]

a = {}
for line in file_a.readlines():
    split_line = line.strip().split('\t')
    a[split_line[0]] = split_line[1:]

edited Jun 11 '14 at 10:50

answered Jun 11 '14 at 10:28

Saimadhav Heblikar

702
5
15

why does that give me 'X': [['1', '2', '3']] with a list within a list? – Mark Jun 11 '14 at 10:37

score 2 · Answer 2 · edited May 23 '17 at 12:08

2

a = {}
for line in file_a:
    split_line = line.strip().split('\t')
    a[split_line[0]] = split_line[1:]

You slicing expression split_line[::-1] evaluates to split_line reversed, because the third parameter is the step (-1 in this case). You want to start at element 1 and go all the way to the end, with the default step of 1. Check this answer for more on slice notation.

edited May 23 '17 at 12:08

Community

1
1

answered Jun 11 '14 at 10:27

Lev Levitsky

63,701
20
147
175

python accessing the second to the last element in a list

2 Answers2