[Python-ideas] Additions to collections.Counter and a Counter derived class

[David Mertz]

On Tue, Mar 14, 2017 at 2:38 AM, Marco Cognetta <cognetta.marco at gmail.com>
wrote:

> 1) Addition of a Counter.least_common method:
> This was addressed in https://bugs.python.org/issue16994, but it was
> never resolved and is still open (since Jan. 2013). This is a small
> change, but I think that it is useful to include in the stdlib.


-1 on adding this.  I read the issue, and do not find a convincing use case
that is common enough to merit a new method.  As some people noted in the
issue, the "least common" is really the infinitely many keys not in the
collection at all.

But I can imagine an occasional need to, e.g. "find outliers."  However,
that is not hard to spell as `mycounter.most_common()[-1*N:]`.  Or if your
program does this often, write a utility function `find_outliers(...)`

2) Undefined behavior when using Counter.most_common:
> 'c', 'c']), when calling c.most_common(3), there are more than 3 "most
> common" elements in c and c.most_common(3) will not always return the
> same list, since there is no defined total order on the elements in c.
>
Should this be mentioned in the documentation?
>

+1. I'd definitely support adding this point to the documentation.


> Additionally, perhaps there is room for a method that produces all of
> the elements with the n highest frequencies in order of their
> frequencies. For example, in the case of c = Counter([1, 1, 1, 2, 2,
> 3, 3, 4, 4, 5]) c.aforementioned_method(2) would return [(1, 3), (2,
> 2), (3, 2), (4, 2)] since the two highest frequencies are 3 and 2.
>

-0 on this.  I can see wanting this, but I'm not sure often enough to add
to the promise of the class.  The utility function to do this would be
somewhat less trivial to write than `find_outliers(..)` but not enormously
hard.  I think I'd be +0 on adding a recipe to the documentation for a
utility function.


> 3) Addition of a collections.Frequency or collections.Proportion class
> derived from collections.Counter:
>
> This is sort of discussed in https://bugs.python.org/issue25478.
> The idea behind this would be a dictionary that, instead of returning
> the integer frequency of an element, would return it's proportional
> representation in the iterable.


One could write a subclass easily enough.  The essential feature in my mind
would be to keep an attributed Counter.total around to perform the
normalization.  I'm +1 on adding that to collections.Counter itself.

I'm not sure if this would be better as an attribute kept directly or as a
property that called `sum(self.values())` when accessed.  I believe that
having `mycounter.total` would provide the right normalization in a clean
API, and also expose easy access to other questions one would naturally ask
(e.g. "How many observations were made?")



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