为什么 'Extended Iterable Unpacking' 不适用于空字符串？

Question

为什么是

>>> a, *b = ''

不可能，当

>>> a, *b = ' '
>>> a, b
(' ', [])  # b == empty list here anyway.

和

>>> type('')
<class 'str'>

我的意思是，为什么不是

>>> a, *b = ''
>>> a, b  # a could == ''
('', [])

Answer 1

因为指定了一个强制变量。

右侧至少应有一项（字符串为一个字符）。

根据PEP-3131：

A tuple (or list) on the left side of a simple assignment (unpacking is not defined for augmented assignment) may contain at most one expression prepended with a single asterisk (which is henceforth called a "starred" expression, while the other expressions in the list are called "mandatory"). This designates a subexpression that will be assigned a list of all items from the iterable being unpacked that are not assigned to any of the mandatory expressions, or an empty list if there are no such items.

Answer 2

基于PEP 3132：

The function unpack_iterable() in ceval.c is changed to handle the extended unpacking, via an argcntafter parameter. In the UNPACK_EX case, the function will do the following:

在加星标之前收集强制性目标的所有项目
从列表中的可迭代项中收集所有剩余项弹出项
列表中加星标后的强制目标推单堆栈上的项目和调整大小的列表

因此第一步会失败，因为您的字符串中没有强制性目标。

有关详细信息，您可以在 caval.c 中查看 unpack_iterable :

unpack_iterable(PyObject *v, int argcnt, int argcntafter, PyObject **sp)
{
    int i = 0, j = 0;
    Py_ssize_t ll = 0;
    PyObject *it;  /* iter(v) */
    PyObject *w;
    PyObject *l = NULL; /* variable list */

    assert(v != NULL);

    it = PyObject_GetIter(v);
    if (it == NULL)
        goto Error;

    for (; i < argcnt; i++) {
        w = PyIter_Next(it);
        if (w == NULL) {
            /* Iterator done, via error or exhaustion. */
            if (!PyErr_Occurred()) {
                if (argcntafter == -1) {
                    PyErr_Format(PyExc_ValueError,
                        "not enough values to unpack (expected %d, got %d)",
                        argcnt, i);
                }
                else {
                    PyErr_Format(PyExc_ValueError,
                        "not enough values to unpack "
                        "(expected at least %d, got %d)",
                        argcnt + argcntafter, i);
                }
            }
            goto Error;
        }
        *--sp = w;
    }

    if (argcntafter == -1) {
        /* We better have exhausted the iterator now. */
        w = PyIter_Next(it);
        if (w == NULL) {
            if (PyErr_Occurred())
                goto Error;
            Py_DECREF(it);
            return 1;
        }
        Py_DECREF(w);
        PyErr_Format(PyExc_ValueError,
            "too many values to unpack (expected %d)",
            argcnt);
        goto Error;
    }

    l = PySequence_List(it);
    if (l == NULL)
        goto Error;
    *--sp = l;
    i++;

    ll = PyList_GET_SIZE(l);
    if (ll < argcntafter) {
        PyErr_Format(PyExc_ValueError,
            "not enough values to unpack (expected at least %d, got %zd)",
            argcnt + argcntafter, argcnt + ll);
        goto Error;
    }

    /* Pop the "after-variable" args off the list. */
    for (j = argcntafter; j > 0; j--, i++) {
        *--sp = PyList_GET_ITEM(l, ll - j);
    }
    /* Resize the list. */
    Py_SIZE(l) = ll - argcntafter;
    Py_DECREF(it);
    return 1;

Error:
    for (; i > 0; i--, sp++)
        Py_DECREF(*sp);
    Py_XDECREF(it);
    return 0;
}

为什么 'Extended Iterable Unpacking' 不适用于空字符串？

Why is 'Extended Iterable Unpacking' not working for empty strings?

python

python-3.x

iterable-unpacking