[Python-ideas] Consider adding clip or clamp function to math

[Chris Barker]

Something to keep in mind:

the math module is written in C, and will remain that way for the time
being (see recent discussion on, I think, this list and also the discussion
when we added math.isclose()

which means it will be for floats only.

My first thought is that not every one line function needs to be in the
standard library. However, as this thread shows, there are some
complications to be considered, so maybe it does make sense to have them
hashed out.

Regarding NaN:

In [4]: nan = float('nan')

In [6]: nan > 5

Out[6]: False

In [7]: 5 > nan

Out[7]: False

This follows the IEEE spec -- so the only correct result from

clip(x, float('nan')) is NaN.

Steven D'Aprano wrote:

I don't care too much whether the parameters are mandatory or have
> defaults, so long as it is *possible* to pass something for the lower
> and upper bounds which mean "unbounded".


I think the point was that if one of the liimts in unbounded, then you can
jsut use min or max...

though I think I agree -- you may have code where the limits are sometimes
unbounded, and sometimes not -- nice to have a way to have only one code
path.

 (1) Explicitly pass -INFINITY or +INFINITY as needed;
but which

that's it then.


> infinity, float or Decimal? If you pass the wrong one, you may have to
> pay the cost of converting your values to float/Decimal, which could end
> up expensive if you have a lot of them.


well, as above, if it's in the math module, it's only float.... you could
add one ot the Decimal module, too, I suppose.


> (2) Pass a NAN as the bounds. With my implementation, that actually
> works! But it's a surprising accident of implementation, it feels wrong
> and looks weird,


and violates IEEE754 -- don't do that.


> (3) Use some special Infimum and Supremum objects which are smaller
> than, and greater than, every other value. But we don't have such
> objects, so you'd need to create your own.
>


that's what float('inf') already is -- let's use them.


> (4) Use None as a placeholder for "no limit". That's my preferred
> option.


reasonable enough -- and would make the API a bit easier -- both for
matching different types, and because there is no literal or pre-existing
object for Inf.

-Chris





On Sun, Jul 31, 2016 at 7:47 PM, Steven D'Aprano <steve at pearwood.info>
wrote:

> On Sun, Jul 31, 2016 at 09:38:44PM +0200, Victor Stinner wrote:
> > I dislike this API. What's the point of calling clamp(x)? clamp(b, a) is
> > min(a, b) and clamp(a, max_val=b) is just max(a, b).
>
> You have that the wrong way around. If you supply a lower-bounds, you
> must take the max(), not the min(). If you supply a upper-bounds, you
> take the min(), not the max(). It's easy to get wrong.
>
>
> > My point is that all parameters must be mandatory.
>
> I don't care too much whether the parameters are mandatory or have
> defaults, so long as it is *possible* to pass something for the lower
> and upper bounds which mean "unbounded". There are four obvious
> alternatives (well three obvious ones and one surprising one):
>
> (1) Explicitly pass -INFINITY or +INFINITY as needed; but which
> infinity, float or Decimal? If you pass the wrong one, you may have to
> pay the cost of converting your values to float/Decimal, which could end
> up expensive if you have a lot of them.
>
> (2) Pass a NAN as the bounds. With my implementation, that actually
> works! But it's a surprising accident of implementation, it feels wrong
> and looks weird, and again, it may require converting the values to
> float/Decimal.
>
> (3) Use some special Infimum and Supremum objects which are smaller
> than, and greater than, every other value. But we don't have such
> objects, so you'd need to create your own.
>
> (4) Use None as a placeholder for "no limit". That's my preferred
> option.
>
> Of course, even if None is accepted as "no limit", the caller can still
> explicitly provide an infinity if they prefer.
>
> As I said, I don't particularly care whether the lower and upper bounds
> have default values. But I think it is useful and elegant to accept None
> (as well as infinity) to mean "no limit".
>
>
>
> --
> Steve
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>



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