Fix PyFloat::py_new always returning new float object issue

[jopemachine]

Fixes #3978.

>>>>> import math
>>>>> a = float(math.nan)
>>>>> b = float(math.nan)
>>>>> a is b
>>>>> true

Reference

• https://github.com/python/cpython/blob/main/Objects/abstract.c#L1633-L1635

• https://github.com/RustPython/RustPython/blob/main/vm/src/builtins/int.rs#L258-L264

[Snowapril]

It seems below incompatibility make extra_test failed.

class F(float):
    def __int__(self):
        return 3
# rustpython
>>>>> type(F(1.2))
<class 'float'>
# cpython
>>> type(F(1.2))
<class '__main__.F'>

[Snowapril]

We need to move the exact float type check into PyNumber::float and return its payload with ref. Please refer to what payload_if_exact return and use into_ref 😊

[jopemachine]

We need to move the exact float type check into PyNumber::float and return its payload with ref. Please refer to what payload_if_exact return and use into_ref 😊

number::float is not called when the argument is math.nan because try_float_opt early returned in this line.

So I tried to edit try_float_opt instead like below, but it seems it is not working as expected. (id(float(math.nan)) prints different oid each time)

pub fn try_float_opt(&self, vm: &VirtualMachine) -> PyResult<Option<PyRef<PyFloat>>> {
	let value = if let Some(float) = self.payload_if_exact::<PyFloat>(vm) {
			Some(float.into_ref(vm))
	} else {
			let number = self.to_number();
			#[allow(clippy::manual_map)]
			if let Some(f) = number.float(vm)? {
					Some(f)
			} else if let Some(i) = self.try_index_opt(vm) {
					let value = int::try_to_float(i?.as_bigint(), vm)?;
					Some(vm.ctx.new_float(value))
			} else if let Some(value) = self.downcast_ref::<PyFloat>() {
					Some(vm.ctx.new_float(value.to_f64()))
			} else {
					None
			}
	};
	Ok(value)
}

I guess this is because below line returned new float instance instead of existing one.

https://github.com/RustPython/RustPython/blob/main/vm/src/builtins/float.rs#L152-L154

So, I think maybe we need to update the py_new function somehow.

[youknowone]

you are right. you will find PyInt have a similar logic you are looking for

[youknowone]

looks good, would you add a related test to extra_tests?

[youknowone]

Thank you!