python - 如果乘以负值，列表或元组如何变为空？

Question

如果乘以负值，列表或元组如何变为空？

例如:

>>> b = [1,2,3] * 2
>>> b
[1, 2, 3, 1, 2, 3]
>>> c = [1,2,3] * -1
>>> c
[]
>>> d = [1,2,3] * -2
>>> d
[]

Answer 1

文档

正如人们在我之前回答的那样，documentation 中定义的 Python 标准行为:

In the latter case, sequence repetition is performed; a negative repetition factor yields an empty sequence.

---

源代码

如果你想知道为什么会这样，你可以关注 CPython 的源代码:

从abstract.c中的代码开始:

PyObject *
PyNumber_Multiply(PyObject *v, PyObject *w)
{
    PyObject *result = binary_op1(v, w, NB_SLOT(nb_multiply));
    ...
        if  (mv && mv->sq_repeat) {
            return sequence_repeat(mv->sq_repeat, v, w);
        }
        else if (mw && mw->sq_repeat) {
            return sequence_repeat(mw->sq_repeat, w, v);
        }
    ...
}

然后，在 listobject.c 上:

static PySequenceMethods list_as_sequence = {
    ...
    (ssizeargfunc)list_repeat,                  /* sq_repeat */
    ...
}

跳转到list_repeat :

static PyObject *
list_repeat(PyListObject *a, Py_ssize_t n)
{
    ...
    if (n < 0)
        n = 0;
    ...
    if (size == 0)
        return PyList_New(0);
    ...
}