Math error in BinaryPower example in User Guide

[tzelleke]

The BinaryPower class in the Specialized Parameter types subsection of the user guide does not validate binary powers but merely divisibility by 2.

https://param.holoviz.org/user_guide/Parameters.html#specialized-parameter-types

Another issue is that this Parameter class does not respect None values.

If you want to keep this example based on param.Parameter you might update it to:

    def _validate_value(self, val, allow_None):
        if allow_None and val is None:
            return
        
        super(BinaryPower, self)._validate_value(val, allow_None)
        if not isinstance(val, numbers.Number):
            raise ValueError("BinaryPower parameter %r must be a number, "
                             "not %r." % (self.name, val))
        
        if (not float(val).is_integer()) or (val < 2) or (bin(int(val)).count("1") is not 1):
            # in Python >= 3.10 use `int(val).bit_count() is not 1`
            raise ValueError("BinaryPower parameter %r must be a power of 2, "
                             "not %r." % (self.name, val))

I would rather suggest to have the example based on param.Integer with default=2... but that does not read so well with the way this section is structured currently ?

[jlstevens]

Thanks for reporting this!

In #634 I address this by keeping the current code and changing the example to one that only allows even integers. The purpose of the docs here is to show that parameters can be defined easily and not to have people think too much about the precise validation logic itself.

[jlstevens]

Addressed in #634 which is now merged.