Unless I've misunderstood this API, and the ByteArray part doesn't refer to an array of bytes representing the value of PyLong in 2's complement, can we implement this in the public header? It's already available, it's just not exported (at least I can't use it, and it's not in the documentation).

This would be beneficial for fixed width integer types, especially ones larger than long long (e.g. __int128_t), and shouldn't take much effort since it's already implemented privately (https://github.com/python/cpython/blob/04fc4f2a46b2fd083639deb872c3a3037fdb4...). Possibly include a function to return the size of the would-be array to be used as a check?

Also since I have nowhere else to ask it, what's with PyLong_AsSignedType doing type coercion for you, but PyLong_AsUnsignedType not? (At least that's what I gathered from the documentation)