网站设计是做什么的,wordpress4.9.5,深圳10大品牌策划公司,郑州公司建网站有这么个问题::t [1, 2, 3]t[1:1] [7]print t # 输出 [1, 7, 2, 3]谁会对列表这么进行赋值呢#xff1f;但是对于这个输出结果的原因确实值得去再了解下#xff0c;今天看看Python的源码#xff0c;了解下原理是什么。注#xff1a;本地下载的是Python2.7.6的代码#…有这么个问题::t [1, 2, 3]t[1:1] [7]print t # 输出 [1, 7, 2, 3]谁会对列表这么进行赋值呢但是对于这个输出结果的原因确实值得去再了解下今天看看Python的源码了解下原理是什么。注本地下载的是Python2.7.6的代码直接看这个。在Objects/listobject.c中有一个 PyList_SetSlice 函数是这么写的::intPyList_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 那么再来看看这个函数的代码::static intlist_ass_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v){/* Because [X]DECREF can recursively invoke list operations onthis list, we must postpone all [X]DECREF activity untilafter the list is back in its canonical shape. Thereforewe must allocate an additional array, recycle, into whichwe temporarily copy the items that are deleted from thelist. :-( */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;/*要赋值的长度n*/n PySequence_Fast_GET_SIZE(v_as_SF);vitem PySequence_Fast_ITEMS(v_as_SF);}if (ilow ilow 0;else if (ilow Py_SIZE(a))ilow Py_SIZE(a);if (ihigh 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 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) goto Error;item a-ob_item;printf(关键点\n);/*把list对应切片后一位的值之后的所有内容向后移动所赋值的大小按照上面的python代码这里就是原理的t:|1|2|3|后移一位因为len([7]) 1|1|空|2|3|把后两个移位*/memmove(item[ihigh d], item[ihigh],(k - ihigh)*sizeof(PyObject *));}/*赋值操作即把[7]赋值到t里的对应位置上ilow是1, n是1*/for (k 0; k 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}源码内有详细注释编程问题的研究最好的解释还是源码。
