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Python中對切片賦值的原理是什么?很多新手對此不是很清楚,為了幫助大家解決這個難題,下面小編將為大家詳細講解,有這方面需求的人可以來學(xué)習(xí)下,希望你能有所收獲。
有這么個問題::
t = [1, 2, 3] t[1:1] = [7] print t # 輸出 [1, 7, 2, 3]
誰會對列表這么進行賦值呢?但是對于這個輸出結(jié)果的原因確實值得去再了解下,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。
在Objects/listobject.c中有一個 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 ,那么再來看看這個函數(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\n"); /* 把list對應(yīng)切片后一位的值之后的所有內(nèi)容向后移動所賦值的大小 按照上面的python代碼這里就是 原理的t: |1|2|3| 后移一位,因為len([7]) = 1 |1|空|2|3|把后兩個移位 */ 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)有詳細注釋,編程問題的研究最好的解釋還是源碼。
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