dict comprehension question.

Mitya Sirenef msirenef at lightbird.net
Sat Dec 29 21:01:04 CET 2012


On 12/29/2012 02:48 PM, 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?
>
> TIA

Will this do?:

 >>> w = [1, 2, 3, 1, 2, 4, 4, 5, 6, 1]
 >>> {x: w.count(x) for x in w}
{1: 3, 2: 2, 3: 1, 4: 2, 5: 1, 6: 1}


  - mitya

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Lark's Tongue Guide to Python: http://lightbird.net/larks/




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