gh-74020: Raise ValueError for negative values converted to unsigned

[serhiy-storchaka]

Converting negative Python integer to a C unsigned integer type now raises ValueError, not OverflowError.

Affected C API: PyLong_AsUnsignedLong(), PyLong_AsUnsignedLongLong(), PyLong_AsSize_t() and the "b" format unit in PyArg_Parse().

• Issue: Raise ValueError rather of OverflowError in PyLong_AsUnsignedLong() #74020

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📚 Documentation preview 📚: https://cpython-previews--121114.org.readthedocs.build/

[skirpichev]

I would expect this breaks a lot of people code.

Lets do this with a deprecation period.

[skirpichev]

You should adjust int.to_bytes() docs (both docstring and sphinx).

BTW, I think there could be a test with a negative input and signed=True.

[serhiy-storchaka]

Updated docs.

There is a test with a negative input and signed=True above.

[skirpichev]

There is a test with a negative input and signed=True above.

Hmm, maybe. I don't see it.

[picnixz]

I'm really worried that now, anyone using PyLong_AsUnsignedLong would need to either change how they handle the exception or now be dealing with two different exceptions.

I'm not confident enough to actually just make this change. OverflowError fits in the sense that it's outside of the expected range, but the transition is too hard. I suggested raising DomainError which would inherit from OverflowError and ValueError but code paths with distinct handling of ValueError and OverflowError could be broken.

An easy way to make it work is to introduce a new function that would properly raise what is meant to be raised, and deprecate those functions.

So I'm not -1 as strong as @rhettinger, but I still think it's better to be continue with this, even if it's annoying on our side. We could have some private helpers that would do something like:

try:
	x = asulong(X)
except OverflowError:
	if X < 0:
		raise ValueError
	raise

Or something that can store in addition the fact that something is < 0 or not, like PyLong_AsLongAndOverflow but for ULong instead where we add also a isnegative flag:

int _PyLong_AsUnsignedLong(PyObject *long, int *overflow, int *negative);

[picnixz]

This will need to be moved to 3.15 and marked as an incompatible change.