flask - python让列表倒序输出

Question

想请问下python中用下面的语法就可以让一个list倒序

list = [1, 2, 3, 4, 5, 6]
print list[::-1]

想请问下其中是什么道理吗，还是就是一种特定用法记住就好了

Answer 1

The slice of list has three parameters: starting point, end point, step size

list[::-1] is equivalent to the starting point being the last one, the end point being the first one, and then decreasing one at a time

See more tests below

>>> a = [0,1,2,3,4,5,6,7,8,9]
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a.reverse()
>>> a
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> a.reverse()
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[:]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[::]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[5::-1]
[5, 4, 3, 2, 1, 0]
>>> a[:5:-1]
[9, 8, 7, 6]
>>> a[5:1:-1]
[5, 4, 3, 2]

Answer 2

I am here to post the document——

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 which end depends on the sign of k). Note , k cannot be zero. If k is None, it is treated like 1.

PS: Once again encountered the problem that Markdown does not support starting a list other than 1...

Answer 3

a = [0,1,2,3,4,5,6,7,8,9]
b = a[i:j] means copying a[i] to a[j-1] to generate a new list object
b = a[1:3] Then, the content of b is [1,2]
When i is defaulted, it defaults to 0, that is, a[:3] is equivalent to a[0:3]
When j is defaulted, it defaults to len(alist), that is, a[1:] is equivalent to a[1:10]
When i and j are both default, a[:] is equivalent to a complete copy of a

b = a[i:j:s] In this format, i and j are the same as above, but s represents step, and the default is 1.
So a[i:j:1] is equivalent to a[i:j]
When s<0, when i is defaulted, the default is -1. When j is defaulted, the default is -len(a)-1
So a[::-1] is equivalent to a[-1:-len(a)-1:-1], that is, copying from the last element to the first element. So you see something in reverse order.

If you still don’t understand, test what I said and you will understand

Answer 4

This is a special usage of python’s slice notation. However, it is recommended not to remember this, it is best to use it like this:

reversed([1, 2, 3, 4, 5])