This callback idea feels way over-engineered for this module. It would absolutely make sense in a more specialized numeric or statistical library. But `statistics` feels to me like it should be only simple and basic operations, with very few knobs attached.

On Mon, Jan 7, 2019, 2:36 PM MRAB < wrote:
On 2019-01-07 16:34, Steven D'Aprano wrote:
> On Mon, Jan 07, 2019 at 10:05:19AM -0500, David Mertz wrote:
>>  It's not hard to manually check for NaNs and
>> generate those in your own code.
> That is correct, but by that logic, we don't need to support *any* form
> of NAN handling at all. It is easy (if inefficent) for the caller to
> pre-filter their data. I want to make it easier and more convenient and
> avoid having to iterate over the data twice if it isn't necessary.
Could the functions optionally accept a callback that will be called
when a NaN is first seen?

If the callback returns False, NaNs are suppressed, otherwise they are
retained and the function returns NaN (or whatever).

The callback would give the user a chance to raise a warning or an
exception, if desired.
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