元组切片在Python中到底如何工作？

Question

For list slicing I could imagine (although I could be totally off...) the code being something along the lines of instantiating a list of size range_end - range_start and inputting values from the original list into the newly created array.

How exactly does tuple slicing work? It's not like we could potentially instantiate a tuple of size range_end - range_start and update the values since tuples are immutable.

I've tried a couple of tests and it seems tuple slicing and list slicing perform similarly on average:

import time

_max = 10000000
_list = range(_max)
_tuple = tuple(_list)

mid = _max // 2
i = 10000
while i <= _max:
    print 'for', i,  ' elements :'
    s = mid - half
    e = mid + half - 1
    # list slice
    start = time.clock()
    lSlice = _list[s:e]
    end = time.clock()
    print 'list slice took:',  end - start
    # tuple slice
    start = time.clock()
    tSlice = _tuple[s:e]
    end = time.clock()
    print 'tuple slice took:', end - start
    i *= 10

Which yielded:

for 10000  elements :
list slice took: 6.78296778938e-05
tuple slice took: 2.94523601381e-05
for 100000  elements :
list slice took: 0.000377971955106
tuple slice took: 0.000270872463694
for 1000000  elements :
list slice took: 0.00472353381912
tuple slice took: 0.00548037022509
for 10000000  elements :
list slice took: 0.0499159492116
tuple slice took: 0.0504157468382

Could anyone give me some insight into how the tuple slicing routine works?

edit: I noticed my setup for start and end points of slices were ridiculous so I updated the code snippet.

Answer 1

It's not like we could potentially instantiate a tuple of size range_end - range_start and update the values since tuples are immutable.

You can't at Python level, but it's entirely possible in the underlying C implementation, and that's how it's done . That's how any tuple gets its values filled in, whether through slicing, the tuple constructor, (a, b, c) syntax, or anything else.

else if (PySlice_Check(item)) {
    ...
        result = PyTuple_New(slicelength);
        if (!result) return NULL;

        src = self->ob_item;
        dest = ((PyTupleObject *)result)->ob_item;
        for (cur = start, i = 0; i < slicelength;
             cur += step, i++) {
            it = src[cur];
            Py_INCREF(it);
            dest[i] = it;
        }

        return result;