How to explain the reverse of a sequence by slice notation a[::-1]

Question

From the python.org tutorial

Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size of the string being sliced.

>>> a = "hello" >>> print(a[::-1]) olleh 

As the tutorial says a[::-1] should equals to a[0:5:-1]

but a[0:5:-1] is empty as follows:

>>> print(len(a[0:5:-1])) 0 

The question is not a duplicate of explain-slice-notation. That question is about the general use of slicing in python.

Answer 1

I think the docs are perhaps a little misleading on this, but the optional arguments of slicing if omitted are the same as using None:

>>> a = "hello" >>> a[::-1] 'olleh' >>> a[None:None:-1] 'olleh' 

You can see that these 2 above slices are identical from the CPython bytecode:

>>> import dis >>> dis.dis('a[::-1]') # or dis.dis('a[None:None:-1]')   1           0 LOAD_NAME                0 (a)               3 LOAD_CONST               0 (None)               6 LOAD_CONST               0 (None)               9 LOAD_CONST               2 (-1)              12 BUILD_SLICE              3              15 BINARY_SUBSCR              16 RETURN_VALUE 

For a negative step, the substituted values for None are len(a) - 1 for the start and -len(a) - 1 for the end:

>>> a[len(a)-1:-len(a)-1:-1] 'olleh' >>> a[4:-6:-1] 'olleh' >>> a[-1:-6:-1] 'olleh' 

This may help you visualize it:

    h  e  l  l  o        0  1  2  3  4  5 -6 -5 -4 -3 -2 -1