Python

Question

Documentation for class slice(start, stop[, step]):

Return a slice object representing the set of indices specified by range(start, stop, step).

代码中发生了什么以及为什么切片 class init 甚至允许列表作为其参数？

print(slice([1,3]))
---
slice(None, [1, 3], None)

print(slice(list((1,3))))
---
slice(None, [1, 3], None)  # why stop is list?

hoge = [1,2,3,4]
_s = slice(list((1,3)))
print(hoge[_s])
--------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-59-1b2df30e9bdf> in <module>
      1 hoge = [1,2,3,4]
      2 _s = slice(list((1,3)))
----> 3 print(hoge[_s])

TypeError: slice indices must be integers or None or have an __index__ method

更新

感谢 Selcuk 的回答。

sliceobject.c#L303-L322

static PyObject *
slice_new(PyTypeObject *type, PyObject *args, PyObject *kw)
{
    PyObject *start, *stop, *step;

    start = stop = step = NULL;

    if (!_PyArg_NoKeywords("slice", kw))
        return NULL;

    if (!PyArg_UnpackTuple(args, "slice", 1, 3, &start, &stop, &step))
        return NULL;

    /* This swapping of stop and start is to maintain similarity with
       range(). */
    if (stop == NULL) {
        stop = start;       // <-----
        start = NULL;
    }
      
    return PySlice_New(start, stop, step);  // PySlice_New in L110 in the same file
}

Answer 1

来自 the documentation:

Slice objects have read-only data attributes start, stop and step which merely return the argument values (or their default). They have no other explicit functionality [...]

所以它们只是虚拟对象，可以保留您传递给它们的任何内容。您甚至可以传递字符串或其他对象：

my_slice = slice("foo", "bar", "baz")

[...] however they are used by Numerical Python and other third party extensions.

第三方扩展的工作是验证 start、stop 和 step 值是否有意义。

另见 CPython implementation。

当您只传递一个参数时，它被假定为 stop 值。这就是为什么您最终将 start 和 step 值设置为 None:

class slice(stop)

class slice(start, stop[, step])

Python - slice([1,2,3]) 是如何工作的以及 slice(None, [1, 3], None) 代表什么？

Python - how slice([1,2,3]) works and what does slice(None, [1, 3], None) represent?

slice

python-3.x

更新