[Python-ideas] collections.Counter should implement __mul__, __rmul__

[Wes Turner]

Good call. Is it any faster to initialize Counter with a dict comprehension?

return Counter({k: v*scalar for (k, v) in self.items())

On Sun, Apr 15, 2018 at 5:05 PM, Peter Norvig <peter at norvig.com> wrote:

> For most types that implement __add__, `x + x` is equal to `2 * x`.
>
> That is true for all numbers, list, tuple, str, timedelta, etc. -- but not
> for collections.Counter. I can add two Counters, but I can't multiply one
> by a scalar. That seems like an oversight.
>
> It would be worthwhile to implement multiplication because, among other
> reasons, Counters are a nice representation for discrete probability
> distributions, for which multiplication is an even more fundamental
> operation than addition.
>
> Here's an implementation:
>
>     def __mul__(self, scalar):
>         "Multiply each entry by a scalar."
>         result = Counter()
>         for key in self:
>             result[key] = self[key] * scalar
>         return result
>
>     def __rmul__(self, scalar):
>         "Multiply each entry by a scalar."
>         result = Counter()
>         for key in self:
>             result[key] = scalar * self[key]
>         return result
>
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