[Python-ideas] statistics.sum [was Re: Pre-PEP: adding a statistics module to Python]

[Ryan]

+99. In Python, if you don't have to put multiple lines of code for a single task, you shouldn't have to. That's my opinion, at least.

What other stuff might this statistics module have? It's own array format like NumPy? I'm curious.

Steven D'Aprano <steve at pearwood.info> wrote:

>On 03/08/13 03:45, Steven D'Aprano wrote:
>> I have raised an issue on the tracker to add a statistics module to
>Python's standard library:
>>
>> http://bugs.python.org/issue18606
>
>
>Thanks to everyone who has given feedback, it has been very humbling
>and informative. I have a revised proto-PEP just about ready for
>(hopefully final) feedback, but before I do there is one potentially
>major stumbling block: whether or not the statistics module should have
>it's own version of sum.
>
>
>Against the idea
>----------------
>
>
>* One Obvious Way / Only One Way -- there are already two ways to
>calculate a sum (builtins.sum and math.fsum), no need for a third.
>
>* Even if there is a need, we should aim to fix the problems with the
>existing sum functions rather than add a third.
>
>* Even if we can't, don't call it "sum", call it something else.
>"precise_sum" was the only suggestion given so far.
>
>(If I have missed any objections, I apologize.)
>
>
>
>In favour
>---------
>
>* Speaking as the module author, it is my considered opinion that I
>cannot (easily, or at all) get the behaviour I expect from the
>statistics module using the existing sum functions without a lot of
>pain. See below.
>
>* For backward compatibility, I don't think we can change the existing
>sum functions. E.g.:
>
>   - built-in sum accepts non-numeric values if they support
>     the + operator, that won't change before Python 4000;
>
>   - built-in sum can be inaccurate with floats;
>
>   - math.fsum coerces everything to float.
>
>* Even if we could change one of the existing sum functions (math.fsum
>is probably the better candidate) I personally don't know enough C to
>do so. Either somebody else steps up and volunteers, or any such change
>is deferred indefinitely.
>
>
>Now that Decimal has an accelerated C version in CPython, it is more
>important that ever before to treat it (and Fraction) as first-class
>numeric types, and avoid coercing them to float unless necessary. So I
>consider it a Must Have that stats functions support Decimal and
>Fraction data without unnecessarily converting them to floats. This
>rules out fsum:
>
>py> from decimal import Decimal as D
>py> data = [D("0.1"), D("0.3")]
>py> math.fsum(data)
>0.4
>py> statistics.sum(data)
>Decimal('0.4')
>
>
>On the other hand, the built-in sum is demonstrably inaccurate with
>floats, which is why fsum exists in the first place:
>
>py> data = [1e100, 1, -1e100, 1]
>py> sum(data)
>1.0
>py> math.fsum(data)
>2.0
>py> statistics.sum(data)
>2.0
>
>
>Consequently the statistics module includes its own version of sum.
>Never mind the implementation, that may change in the future.
>Regardless of the implementation, the interface of statistics.sum is
>distinct from both of the existing sum functions. There are three
>versions of sum because they each do different things.
>
>Currently, I can do this, both internally within other functions such
>as mean, and externally, when I just want a total:
>
>total = statistics.sum(data)
>
>and get the right result regardless of the numeric type of data[1].
>Without it, I have to do something like this:
>
># Make sure data is a list, and not an iterator.
>if any(isinstance(x, float) for x in data):
>     total = math.fsum(data)
>else:
>     total = sum(data)
>
>
>Are there still objections to making statistics.sum public? If the only
>way to move forward is to make it a private implementation detail, I
>will do so, but I really think that I have built a better sum and hope
>to keep it as a public function.
>
>Show of hands please, +1 or -1 on statistics.sum.
>
>
>
>
>[1] Well, not complex numbers.
>
>-- 
>Steven
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-- 
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