> Or maybe even better would be PyLong_FromBytes(&myinteger, sizeof(myinteger)) ?

There is a private function:

obj = _PyLong_FromByteArray((const unsigned char*)&myinteger, sizeof(myinteger), PY_LITTLE_ENDIAN, is_signed);

Where PY_LITTLE_ENDIAN is 1 on little-endian platform, and is_signed should be 0 or 1.

This function is inefficient (slow). It would be better to have a functions for intmax_t and uintmax_t types. Such functions would be enough to support other "recent" C types: intptr_t, uintptr_t, ptrdiff_t, etc. We may also add support for these types in PyArg_Parse* (ex: with "j" and "J" types, intmax_t and uintmax_t).