Issue with use of typing.cast in get_wt_fraction

[CompRhys]

Python version

py312

Pymatgen version

latest

Operating system version

macos

Current behavior

def get_wt_fraction(self, el: SpeciesLike) -> float:
        """Calculate weight fraction of an Element or Species.

        Args:
            el (Element | Species): Element or Species to get fraction for.

        Returns:
            float: Weight fraction for element el in Composition.
        """
        el_mass = cast("float", get_el_sp(el).atomic_mass)
        return el_mass * abs(self[el]) / self.weight

the typing cast here doesn't change the output to float it's still a float with unit and that caused a serialization error for us in a downstream third party library as we passed a dict of wt_fractions expecting them to be floats from the type hint.

Expected Behavior

It should either be typed as FloatwithUnit or be converted to a float.

Minimal example

Relevant files to reproduce this bug

No response

[DanielYang59]

I guess as a float might be more reasonable as it's a fraction so shouldn't come with a unit?

[CompRhys]

cast("float", get_el_sp(el).atomic_mass) -> float(get_el_sp(el).atomic_mass) is the change I would make

[DanielYang59]

cast("float", get_el_sp(el).atomic_mass) -> float(get_el_sp(el).atomic_mass) is the change I would make

I think this works now, but shouldn't work I assume as self.weight is also FloatWithUnit:

https://github.com/materialsproject/pymatgen/blob/master/src/pymatgen/core/composition.py#L574-L575

Looks like currently calculating the inverse of a FloatWithUnit would drop the unit which is a bit unexpected:

from pymatgen.core.units import FloatWithUnit

a = FloatWithUnit(1, "g")

print(type(a * 1))
print(type(a / 1))
print(type(1 / a))

<class 'pymatgen.core.units.FloatWithUnit'>
<class 'pymatgen.core.units.FloatWithUnit'>
<class 'float'>

I guess the unit should be 1/g in this case instead of being dropped? I would try to fix this later (in another PR as it's not related to this issue here)