Python dictionary comprehension example

Question

I am trying to learn Python dictionary comprehension, and I think it is possible to do in one line what the following functions do. I wasn't able to make the n+1 as in the first or avoid using range() as in the second.

Is it possible to use a counter that automatically increments during the comprehension, as in test1()?

def test1():
    l = ['a', 'b', 'c', 'd']
    d = {}
    n = 1
    for i in l:
        d[i] = n
        n = n + 1
    return d

def test2():
    l = ['a', 'b', 'c', 'd']
    d = {}
    for n in range(len(l)):
        d[l[n]] = n + 1
    return d

Is it okay to use `range` in the `dict` comprehension? – ersran9 Jul 07 '13 at 16:26 — ersran9, Jul 07 '13 at 16:26

Bakuriu · Accepted Answer · 2015-03-04T21:41:27.080

10

It's quite simple using the enumerate function:

>>> L = ['a', 'b', 'c', 'd']
>>> {letter: i for i,letter in enumerate(L, start=1)}
{'a': 1, 'c': 3, 'b': 2, 'd': 4}

Note that, if you wanted the inverse mapping, i.e. mapping 1 to a, 2 to b etc, you could simply do:

>>> dict(enumerate(L, start=1))
{1: 'a', 2: 'b', 3: 'c', 4: 'd'}

edited Mar 04 '15 at 21:41

answered Jul 07 '13 at 16:28

Bakuriu

98,325
22
197
231

score 1 · Answer 2 · answered Jul 07 '13 at 16:23

1

This works

>>> l = ['a', 'b', 'c', 'd']
>>> { x:(y+1) for (x,y) in zip(l, range(len(l))) }
{'a': 1, 'c': 3, 'b': 2, 'd': 4}

answered Jul 07 '13 at 16:23

ersran9

968
1
8
13

Thanks, I didn't know `zip()`. Is it ok to use `range()` inside a comprehension? – stenci Jul 07 '13 at 16:28
1

@stenci sure, you can use range inside comprehensions – ersran9 Jul 07 '13 at 16:28
1

If you don't want to use range, use @Bakuriu's enumerate solution. – ersran9 Jul 07 '13 at 16:29

Python dictionary comprehension example

2 Answers2

Linked