sequence = [1,2,3,4,5] sequence [ilow:ihigh] # 从ilow开始到ihigh-1结束 sequence [ilow:] # 从ilow开始直到末尾 sequence [:ihigh] # 从头部开始直到ihigh结束 sequence [:] # 复制整个列表
sequence = [1,2,3,4,5] print sequence[1] #输出2 print sequence[2] # 输出3
sequence = [1,2,3,4,5] print sequence[15] ###输出 ### Traceback (most recent call last): File "test.py", line 2, in <module> print a[20] IndexError: list index out of range
# version: python2.7 a = [1, 2, 3, 5] print a[10:20] # 结果会报异常吗?
>>> a = [1, 2, 3, 5] >>> print a[10:20] []
>>> s = '23123123123' >>> print s[400:2000] '' >>> t = (1, 2, 3,4) >>> print t[200: 1000] ()
############# 切片 ################
[root@iZ23pynfq19Z ~]# cat test.py
a = [11,2,3,4]
print a[20:30]
#结果:
[root@iZ23pynfq19Z ~]# python -m dis test.py
1 0 LOAD_CONST 0 (11)
3 LOAD_CONST 1 (2)
6 LOAD_CONST 2 (3)
9 LOAD_CONST 3 (4)
12 BUILD_LIST 4
15 STORE_NAME 0 (a)
2 18 LOAD_NAME 0 (a)
21 LOAD_CONST 4 (20)
24 LOAD_CONST 5 (30)
27 SLICE+3
28 PRINT_ITEM
29 PRINT_NEWLINE
30 LOAD_CONST 6 (None)
33 RETURN_VALUE
############# 单下标取值 ################
[root@gitlab ~]# cat test2.py
a = [11,2,3,4]
print a[20]
#结果:
[root@gitlab ~]# python -m dis test2.py
1 0 LOAD_CONST 0 (11)
3 LOAD_CONST 1 (2)
6 LOAD_CONST 2 (3)
9 LOAD_CONST 3 (4)
12 BUILD_LIST 4
15 STORE_NAME 0 (a)
2 18 LOAD_NAME 0 (a)
21 LOAD_CONST 4 (20)
24 BINARY_SUBSCR
25 PRINT_ITEM
26 PRINT_NEWLINE
27 LOAD_CONST 5 (None)
30 RETURN_VALUE /*取自: python2.7 python/ceval.c */
// 第一步:
PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
{
.... // 省略n行代码
TARGET_WITH_IMPL_NOARG(SLICE, _slice)
TARGET_WITH_IMPL_NOARG(SLICE_1, _slice)
TARGET_WITH_IMPL_NOARG(SLICE_2, _slice)
TARGET_WITH_IMPL_NOARG(SLICE_3, _slice)
_slice:
{
if ((opcode-SLICE) & 2)
w = POP();
else
w = NULL;
if ((opcode-SLICE) & 1)
v = POP();
else
v = NULL;
u = TOP();
x = apply_slice(u, v, w); // 取出v: ilow, w: ihigh, 然后调用apply_slice
Py_DECREF(u);
Py_XDECREF(v);
Py_XDECREF(w);
SET_TOP(x);
if (x != NULL) DISPATCH();
break;
}
.... // 省略n行代码
}
// 第二步:
apply_slice(PyObject *u, PyObject *v, PyObject *w) /* return u[v:w] */
{
PyTypeObject *tp = u->ob_type;
PySequenceMethods *sq = tp->tp_as_sequence;
if (sq && sq->sq_slice && ISINDEX(v) && ISINDEX(w)) { // v,w的类型检查,要整型/长整型对象
Py_ssize_t ilow = 0, ihigh = PY_SSIZE_T_MAX;
if (!_PyEval_SliceIndex(v, &ilow)) // 将v对象再做检查, 并将其值转换出来,存给ilow
return NULL;
if (!_PyEval_SliceIndex(w, &ihigh)) // 同上
return NULL;
return PySequence_GetSlice(u, ilow, ihigh); // 获取u对象对应的切片函数
}
else {
PyObject *slice = PySlice_New(v, w, NULL);
if (slice != NULL) {
PyObject *res = PyObject_GetItem(u, slice);
Py_DECREF(slice);
return res;
}
else
return NULL;
}
// 第三步:
PySequence_GetSlice(PyObject *s, Py_ssize_t i1, Py_ssize_t i2)
{
PySequenceMethods *m;
PyMappingMethods *mp;
if (!s) return null_error();
m = s->ob_type->tp_as_sequence;
if (m && m->sq_slice) {
if (i1 < 0 || i2 < 0) {
if (m->sq_length) {
// 先做个简单的初始化, 如果左右下表小于, 将其加上sequence长度使其归为0
Py_ssize_t l = (*m->sq_length)(s);
if (l < 0)
return NULL;
if (i1 < 0)
i1 += l;
if (i2 < 0)
i2 += l;
}
}
// 真正调用对象的sq_slice函数, 来执行切片的操作
return m->sq_slice(s, i1, i2);
} else if ((mp = s->ob_type->tp_as_mapping) && mp->mp_subscript) {
PyObject *res;
PyObject *slice = _PySlice_FromIndices(i1, i2);
if (!slice)
return NULL;
res = mp->mp_subscript(s, slice);
Py_DECREF(slice);
return res;
}
return type_error("'%.200s' object is unsliceable", s); // 字符串对象 StringObject.c: (ssizessizeargfunc)string_slice, /*sq_slice*/ // 列表对象 ListObject.c: (ssizessizeargfunc)list_slice, /* sq_slice */ // 元组 TupleObject.c: (ssizessizeargfunc)tupleslice, /* sq_slice */
/* 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;
} 结论
从上面的sq_slice函数对应的切片函数可以看到, 如果在使用切片时, 左右下标都大于sequence的长度时, 都将会被重新赋值成sequence的长度, 所以咱们一开始的切片: print a[10:20] , 实际上运行的是: print a4:4 . 通过这次的分析, 以后在遇到下标大于对象长度的切片, 应该不会再懵逼了~
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