I lack the relevant experience to have an opinion on most of this, but FWIW
"PyLong_FromBytes/PyLong_ToBytes' seems clearest to me out of the options
proposed.
On Sat, Aug 7, 2021 at 2:23 PM Serhiy Storchaka
Python integers have arbitrary precision. For serialization and interpolation with other programs and libraries we need to represent them as fixed-width integers (little- and big-endian, signed and unsigned). In Python, we can use struct, array, memoryview and ctypes use for some standard sizes and int methods int.to_bytes and int.from_bytes for non-standard sizes. In C, there is the C API for converting to/from C types long, unsigned long, long long and unsigned long long. For other C types (signed and unsigned char, short, int) we need to use the C API for converting to long, and then truncate to the destination type with checking for overflow. For integers type aliases like pid_t we need to determine their size and signess and use corresponding C API or wrapper. For non-standard integers (e.g. 24-bit), integers wider than long long, and arbitrary precision integers all is much more complicated. There are private C API functions _PyLong_AsByteArray and _PyLong_FromByteArray, but they are for internal use only.
I am planning to add public analogs of these private functions, but more powerful and convenient.
PyObject *PyLong_FromBytes(const void *buf, Py_ssize_t size, int byteorder, int signed)
Py_ssize_t PyLong_AsBytes(PyObject *o, void *buf, Py_ssize_t n, int byteorder, int signed, int *overflow)
PyLong_FromBytes() returns the int object. It only fails in case of memory error or incorrect arguments (e.g. buf is NULL).
PyLong_AsBytes() writes bytes to the specified buffer, it does not allocate memory. If buf is NULL it returns the minimum size of the buffer for representing the integer. -1 is returned on error. if overflow is NULL, then OverfowError is raised, otherwise *overflow is set to +1 for overflowing the upper limit, -1 for overflowing the lower limit, and 0 for no overflow.
Now I have some design questions.
1. How to encode the byte order?
a) 1 -- little endian, 0 -- big endian b) 0 -- little endian, 1 -- big endian c) -1 -- little endian, +1 -- big endian, 0 -- native endian.
Do we need to reserve some values for mixed endians?
2. How to specify the reduction modulo 2**(8*size) (like in PyLong_AsUnsignedLongMask)?
Add yet one flag in PyLong_AsBytes()? Use special value for the signed argument? 0 -- unsigned, 1 -- signed, 2 (or -1) -- modulo. Or use some combination of signed and overflow?
3. How to specify saturation (like in PyNumber_AsSsize_t())? I.e. values less than the lower limit are replaced with the lower limit, values greater than the upper limit are replaced with the upper limit.
Same options as for (2): separate flag, encode in signed (but we need two values here) or combination of other parameters.
4. What exact names to use?
PyLong_FromByteArray/PyLong_AsByteArray, PyLong_FromBytes/PyLong_AsBytes, PyLong_FromBytes/PyLong_ToBytes?
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