dict comprehension question.

[MRAB]

On 2012-12-29 19:48, Quint Rankid wrote:
> Newbie question.  I've googled a little and haven't found the answer.
>
> Given a list like:
> w = [1, 2, 3, 1, 2, 4, 4, 5, 6, 1]
> I would like to be able to do the following as a dict comprehension.
> a = {}
> for x in w:
>      a[x] = a.get(x,0) + 1
> results in a having the value:
> {1: 3, 2: 2, 3: 1, 4: 2, 5: 1, 6: 1}
>
> I've tried a few things
> eg
> a1 = {x:self.get(x,0)+1 for x in w}
> results in error messages.
>
> And
> a2 = {x:a2.get(x,0)+1 for x in w}
> also results in error messages.
>
> Trying to set a variable to a dict before doing the comprehension
> a3 = {}
> a3 = {x:a3.get(x,0)+1 for x in w}
> gets this result, which isn't what I wanted.
> {1: 1, 2: 1, 3: 1, 4: 1, 5: 1, 6: 1}
>
> I'm not sure that it's possible to do this, and if not, perhaps the
> most obvious question is what instance does the get method bind to?
>
You can't do it with a comprehension.

The best way is probably with the 'Counter' class:

 >>> w = [1, 2, 3, 1, 2, 4, 4, 5, 6, 1]
 >>> from collections import Counter
 >>> Counter(w)
Counter({1: 3, 2: 2, 4: 2, 3: 1, 5: 1, 6: 1})

If you want the result be a dict, then just the result to 'dict':

 >>> dict(Counter(w))
{1: 3, 2: 2, 3: 1, 4: 2, 5: 1, 6: 1}