切片python字符串时 为何不会引起下标越界?

Question

>>> s = 'qwertyur'
>>> s[10]
Traceback (most recent call last):
  File "<pyshell#26>", line 1, in <module>
    s[10]
IndexError: string index out of range
>>> s[10:100]
''

如上所示

Answer 1

下面是python3中对于切片的解释：

s[i:j]

The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j.
If i or j is greater than len(s), use len(s).
If i is omitted or None, use 0.
If j is omitted or None, use len(s).
If i is greater than or equal to j, the slice is empty.

文档：sequence-types-list-tuple-range
源码：stringobject.c

Answer 2

这里涉及到切片对象slice：

a = '0123456789'
r = slice(-100, 100)  # -100:100
a_new = a[r] # 取缺省值a[:]
print(a_new, a[:])

r = slice(100, 200)  # 100:200
a_new = a[r] # 取缺省值a[len(a):len(a)]
print(a_new, a[len(a):len(a)])

list 中的具体处理方式：

/* 取自ListObject.c */
static PyObject *
list_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh)
{
    PyListObject *np;
    PyObject **src, **dest;
    Py_ssize_t i, len;
    if (ilow < 0)
        ilow = 0;
    else if (ilow > Py_SIZE(a))               // 如果ilow大于a长度, 那么重新赋值为a的长度
        ilow = Py_SIZE(a);
    if (ihigh < ilow)        
        ihigh = ilow;
    else if (ihigh > Py_SIZE(a))              // 如果ihigh大于a长度, 那么重新赋值为a的长度  
        ihigh = Py_SIZE(a);
    len = ihigh - ilow;
    np = (PyListObject *) PyList_New(len);    // 创建一个ihigh - ilow的新列表对象
    if (np == NULL)
        return NULL;

    src = a->ob_item + ilow;
    dest = np->ob_item;
    for (i = 0; i < len; i++) {               // 将a处于该范围内的成员, 添加到新列表对象
        PyObject *v = src[i];
        Py_INCREF(v);
        dest[i] = v;
    }
    return (PyObject *)np;
}