Looking at stringobject.c:1034:

i = 0;  
if (i < size) {  
        Py_MEMCPY(op->ob_sval, a->ob_sval, Py_SIZE(a));  
        i = Py_SIZE(a);  
}  
while (i < size) {  
        j = (i <= size-i)  ?  i  :  size-i;  
        Py_MEMCPY(op->ob_sval+i, op->ob_sval, j);  
        i += j;  
}  
return (PyObject *) op;

Do I miss something or the first condition "if (i < size)" is
a fancy way to check for size > 0?

Wouldn't it be clearer to write

if (size == 0)
        return (PyObject *) op;
Py_MEMCPY(op->ob_sval, a->ob_sval, Py_SIZE(a)); 
i = Py_SIZE(a);
..