Repository navigation
Let math.nextafter() compute multiple steps at a time. #94906
Description
Activity
- added a commit that references this issue
on Jul 16, 2022 This feels like a needless expansion of the API to me; I don't think I've ever encountered a use-case for this. On the rare occasions that such a use-case turns up, what's wrong with the
forloop, or with adding an integer multiple ofmath.ulp(x)tox?-1 from me.
Reacted by Serhiy Storchaka and SycamoreLeafI wouldn't posted the issue if I hadn't needed this, so it is not "needless".
In 30 seconds of searching, I found another example:
Python is a high level language and it is suitable to incorporate options that are more expansive than used in low level languages like C that tend to focus on atomic steps.
Reacted by Erdem ÖzgenSure, I don't doubt that you had a genuine need, but that need is trivially addressed with a
forloop. My use of "needless" referred to the expansion of the API. You needing the functionality is not the same thing as themathmodule needing to support the functionality directly.Sorry, but I don't see this use-case as valuable enough or common enough to warrant expanding the
math.nextafterAPI.I'm also concerned that this might be a performance trap: a user could easily write
nextafter(x, y, steps=10**6)without realising that that means a million invocations ofnextafterinternally. With an explicit for loop, the effect on running time is much more obvious.I concur with @mdickinson. It is too niche feature, and it is not difficult to implement a wrapper in Python. BTW, the case you found is in tests.
Reacted by SycamoreLeafI wish this wasn't dismissed so casually. All uses of nextafter() are "niche". There are only a handful of people who will ever use it.
I submitted the feature request because I needed to write the replacement function in pure Python (much like the example I posted above) and it was distracting and felt amiss, like something the should have already have been built in to a higher level language.
I wrote the function while working on another problem and had casually created an incorrect result along the way:
>>> sixth_largest_random = 1.0 - 6 * ulp(1.0) >>> sixth_largest_random 0.9999999999999987Fortunately, I spotted the problem before going too far with it and wrote the above function giving the correct answer:
>>> sixth_largest_random = newnextafter(1.0, -inf, n=6) >>> sixth_largest_random 0.9999999999999993The entire side trip was distracting and annoying. It took the focus away for the error analysis I was working on at the time.
The proposal is an easy thing to do. It isn't even slightly confusing. It would be handy when needed by the few who ever use this function. I don't see any downside.
Note, in numpy and scipy, most interesting functions have many options. That seems to work well for them. But in Python, there seems to be an urge to fight against the simplest of options as "unnecessary API expansion".
Reacted by Handell and Goncalo Gomes@rhettinger @hauntsaninja @mdickinson
You don't need a loop for this. See this pure Python prototype inspired by an old StackOverflow question of mine.
And here's a prototype of a fix of the PR. (Please keep in mind that the C code in the PR isn't tested nor even run. The pure Python version has Hypothesis tests, so it's much more reliable.)
Reacted by Goncalo Gomes, amirsharif and Stefan PochmannRemoving "easy" for now as this needs a resolution between the two maintainers of the
mathmodule.If the goal here is to produce a one line implementation capable of looping the execution of math.next after, it can be achieved using the reduce function from func tools.
from math import nextafter,inf from functools import reduce reduce(nextafter,[-inf]*6,1.0)The starting point, 1.0, is the third argument, then you must provide an iterator that outputs the direction of nextafter for the desired number of times, in this case I use a list and duplicate -inf 6 times.
7 remaining items
Removing "easy" for now as this needs a resolution between the two maintainers of the math module.
FWIW, I only care about this a tiny bit. Though I think it would be nice to have, I won't lose any sleep if this gets closed.
I overhauled the C PR https://github.com/matthiasgoergens/cpython/pull/5/files Tests pass now.
I'll point it at the upstream CPython repository, after I polish the git history.
- added 4 commits that reference this issue
on Apr 26, 2023 - added a commit that references this issue
on May 19, 2023 - added a commit that references this issue
on May 20, 2023 Done in #103881. Thanks to @matthiasgoergens.
Done in #103881. Thanks to @matthiasgoergens.
Thanks for fixing the PR up at the end, too!
Sometimes
math.nextafter()needs to be applied multiple times in succession.It would be nice if the function supported this directly:
The implementation would just be a for-loop:
The formal paramater can be just
nor the longer but more descriptivesteps.Linked PRs