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 <storch...@gmail.com> wrote: > 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? > > > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/V2EKXMKSQV25BMRPMDH47IM2OYCLY2TF/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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