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創(chuàng)新互聯(lián)Python教程：Python中對(duì)切片賦值原理分析
有這么個(gè)問(wèn)題::
t = [1, 2, 3] t[1:1] = [7] print t # 輸出 [1, 7, 2, 3]
誰(shuí)會(huì)對(duì)列表這么進(jìn)行賦值呢？但是對(duì)于這個(gè)輸出結(jié)果的原因確實(shí)值得去再了解下，今天看看python的源碼，了解下原理是什么。
注：本地下載的是Python2.7.6的代碼，直接看這個(gè)。
在Objects/listobject.c中有一個(gè) PyList_SetSlice 函數(shù)，是這么寫的::
int
PyList_SetSlice(PyObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
{
if (!PyList_Check(a)) {
PyErr_BadInternalCall();
return -1;
}
return list_ass_slice((PyListObject *)a, ilow, ihigh, v);
}
有用的一句就是 list_ass_slice ，那么再來(lái)看看這個(gè)函數(shù)的代碼::
static int
list_ass_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
{
/* Because [X]DECREF can recursively invoke list operations on
this list, we must postpone all [X]DECREF activity until
after the list is back in its canonical shape. Therefore
we must allocate an additional array, 'recycle', into which
we temporarily copy the items that are deleted from the
list. :-( */
PyObject *recycle_on_stack[8];
PyObject **recycle = recycle_on_stack; /* will allocate more if needed */
PyObject **item;
PyObject **vitem = NULL;
PyObject *v_as_SF = NULL; /* PySequence_Fast(v) */
Py_ssize_t n; /* # of elements in replacement list */
Py_ssize_t norig; /* # of elements in list getting replaced */
Py_ssize_t d; /* Change in size */
Py_ssize_t k;
size_t s;
int result = -1; /* guilty until proved innocent */
#define b ((PyListObject *)v)
if (v == NULL)
n = 0;
else {
if (a == b) {
/* Special case "a[i:j] = a" -- copy b first */
v = list_slice(b, 0, Py_SIZE(b));
if (v == NULL)
return result;
result = list_ass_slice(a, ilow, ihigh, v);
Py_DECREF(v);
return result;
}
v_as_SF = PySequence_Fast(v, "can only assign an iterable");
if(v_as_SF == NULL)
goto Error;
/*
要賦值的長(zhǎng)度n
*/
n = PySequence_Fast_GET_SIZE(v_as_SF);
vitem = PySequence_Fast_ITEMS(v_as_SF);
}
if (ilow < 0)
ilow = 0;
else if (ilow > Py_SIZE(a))
ilow = Py_SIZE(a);
if (ihigh < ilow)
ihigh = ilow;
else if (ihigh > Py_SIZE(a))
ihigh = Py_SIZE(a);
norig = ihigh - ilow;
assert(norig >= 0);
d = n - norig;
if (Py_SIZE(a) + d == 0) {
Py_XDECREF(v_as_SF);
return list_clear(a);
}
item = a->ob_item;
/* recycle the items that we are about to remove */
s = norig * sizeof(PyObject *);
if (s > sizeof(recycle_on_stack)) {
recycle = (PyObject **)PyMem_MALLOC(s);
if (recycle == NULL) {
PyErr_NoMemory();
goto Error;
}
}
memcpy(recycle, &item[ilow], s);
if (d < 0) { /* Delete -d items */
memmove(&item[ihigh+d], &item[ihigh],
(Py_SIZE(a) - ihigh)*sizeof(PyObject *));
list_resize(a, Py_SIZE(a) + d);
item = a->ob_item;
}
else if (d > 0) { /* Insert d items */
k = Py_SIZE(a);
if (list_resize(a, k+d) < 0)
goto Error;
item = a->ob_item;
printf("關(guān)鍵點(diǎn)\n");
/*
把list對(duì)應(yīng)切片后一位的值之后的所有內(nèi)容向后移動(dòng)所賦值的大小
按照上面的python代碼這里就是
原理的t:
|1|2|3|
后移一位，因?yàn)閘en([7]) = 1
|1|空|2|3|把后兩個(gè)移位
*/
memmove(&item[ihigh+d], &item[ihigh],
(k - ihigh)*sizeof(PyObject *));
}
/*
賦值操作，即把[7]賦值到t里的對(duì)應(yīng)位置上
ilow是1, n是1
*/
for (k = 0; k < n; k++, ilow++) {
PyObject *w = vitem[k];
Py_XINCREF(w);
item[ilow] = w;
}
for (k = norig - 1; k >= 0; --k)
Py_XDECREF(recycle[k]);
result = 0;
Error:
if (recycle != recycle_on_stack)
PyMem_FREE(recycle);
Py_XDECREF(v_as_SF);
return result;
#undef b
}
源碼內(nèi)有詳細(xì)注釋，編程問(wèn)題的研究最好的解釋還是源碼。
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