Reversing a list using slice notation

in the following example:

foo = ['red', 'white', 'blue', 1, 2, 3]

where: foo[0:6:1] will print all elements in foo. However, foo[6:0:-1] will omit the 1st or 0th element.

>>> foo[6:0:-1]
[3, 2, 1, 'blue', 'white']

I understand that I can use foo.reverse() or foo[::-1] to print the list in reverse, but I'm trying to understand why foo[6:0:-1] doesn't print the entire list?

Solution 1:

Slice notation in short:

[ <first element to include> : <first element to exclude> : <step> ]

If you want to include the first element when reversing a list, leave the middle element empty, like this:

foo[::-1]

You can also find some good information about Python slices in general here:
Explain Python's slice notation

Solution 2:

If you are having trouble remembering slice notation, you could try doing the Hokey Cokey:

[In: Out: Shake it all about]

[First element to include: First element to leave out: The step to use]

YMMV

Solution 3:

...why foo[6:0:-1] doesn't print the entire list?

Because the middle value is the exclusive, rather than inclusive, stop value. The interval notation is [start, stop).

This is exactly how [x]range works:

>>> range(6, 0, -1)
[6, 5, 4, 3, 2, 1]

Those are the indices that get included in your resulting list, and they don't include 0 for the first item.

>>> range(6, -1, -1)
[6, 5, 4, 3, 2, 1, 0]

Another way to look at it is:

>>> L = ['red', 'white', 'blue', 1, 2, 3]
>>> L[0:6:1]
['red', 'white', 'blue', 1, 2, 3]
>>> len(L)
6
>>> L[5]
3
>>> L[6]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

The index 6 is beyond (one-past, precisely) the valid indices for L, so excluding it from the range as the excluded stop value:

>>> range(0, 6, 1)
[0, 1, 2, 3, 4, 5]

Still gives you indices for each item in the list.

Solution 4:

This answer might be a little outdated, but it could be helpful for someone who stuck with same problem. You can get reverse list with an arbitrary end - up to 0 index, applying second in-place slice like this:

>>> L = list(range(10))
>>> L
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> (start_ex, end) = (7, 0)
>>> L[end:start_ex][::-1]
[6, 5, 4, 3, 2, 1, 0]

Solution 5:

You can get it to work if you use a negative stop value. Try this:

foo[-1:-7:-1]