以x [0：4：-1]的形式将元组切出索引

Question

x = (1, 2, 3, 4)

Here, x[0:4:-1] gives an empty tuple () . Why is this happening? I thought it would just give a reversed tuple (4,3,2,1) ...

Answer 1

You're asking for all of the values starting at 0, ending before 4, counting by -1 at a time. That's no values.

What you want to do is start at 3, end before -1, counting by -1 at a time. Except… you can't put -1 in there, because that means "1 from the end". So, you have to write "start at 3, end when you've exhausted the whole sequence, counting by -1 at a time", like this:

x[3::-1]

Or, more simply:

x[::-1]

---

It may help your understanding to turn the slice into an explicit loop. It looks something like this:

def slicify(sequence, start, end, step):
    if start < 0: start += len(sequence)
    if end < 0: end += len(sequence)
    result = []
    while (start < end if step > 0 else start > end):
        result.append(sequence[start])
        start += step
    return result

But this is only roughly correct. The exact definition is in the documentation under Sequence Types , under note 5:

The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k such that 0 <= n < (ji)/k . In other words, the indices are i , i+k , i+2*k , i+3*k and so on, stopping when j is reached (but never including j ). If i or j is greater than len(s) , use len(s) . If i or j are omitted or None , they become “end” values (which end depends on the sign of k ). Note, k cannot be zero. If k is None , it is treated like 1 .

Answer 2

You'd need to either omit the start and end, or reverse the start and end:

x[::-1]    # (4, 3, 2, 1)
x[3:0:-1]  # (4, 3, 2)
x[3::-1]   # (4, 3, 2, 1)
x[3:-5:-1] # (4, 3, 2, 1)

The end point is not included, so slicing with [3:0:-1] only returns three elements. The last example uses a negative value to be subtracted from the length of the tuple to end up with endpoint -1 .

Using a negative stride means Python wants to count backwards, and starting at 0 you'll never get to 4.

Note that the Python slice syntax applies to more than just tuples; strings and lists support the exact same syntax.