Py_ssize_t from_name_len;

static const char *_keywords[] = {"from_name", "strip_digits", NULL};
static _PyArg_Parser _parser = {"s#|$O&:flip_name", _keywords, 0};
if (!_PyArg_ParseTupleAndKeywordsFast(
        args, kw, &_parser, &from_name, &from_name_len, PyC_ParseBool, &strip_digits)) {
  return NULL;
}

==2372449==ERROR: AddressSanitizer: requested allocation size 0x7fff00000008 (0x7fff00001008 after adjustments for alignment, red zones etc.) exceeds maximum supported size of 0x10000000000 (thread T0)
    #0 0x7ffff7682517 in malloc (/lib/x86_64-linux-gnu/libasan.so.6+0xb0517)
    #1 0x1f2ea7f7 in _PyObject_Malloc Objects/obmalloc.c:1645
    #2 0x1f2ea7f7 in _PyObject_Malloc Objects/obmalloc.c:1638

size_t size = from_name_len + 2;
char *name_flip_str = PyMem_MALLOC(size);

BLI_string_flip_side_name(name_flip_str, from_name, strip_number, size);

PyObject *return_value = PyUnicode_FromStringAndSize(name_flip_str, size);

>>> bpy.utils.flip_name('a.Right')
'a.Left\x00\x00p'

>>> bpy.utils.flip_name('a.Left')
'a.Right\x00'

>>> bpy.utils.flip_name('a.L')
Traceback (most recent call last):
  File "<blender_console>", line 1, in <module>
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 4: invalid start byte

>>> bpy.utils.flip_name('a.R')
Traceback (most recent call last):
  File "<blender_console>", line 1, in <module>
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 4: invalid start byte

const size_t name_flip_str_len = BLI_string_flip_side_name(name_flip_str, from_name, strip_digits, size);

PyObject *return_value = PyUnicode_FromStringAndSize(name_flip_str, name_flip_str_len);

PyMem_FREE(name_flip_str);

return return_value;