Why does list[::-1] not equal list[:len(list):-1]?

7
votes

When slicing in python, omitting the end portion of the slice (ie the end in list[:end:]) results in end being defined as "the size of the string being sliced." *

However, this doesn't seem to hold true when using the step argument (the step in list[::step]) in a slice, at least when the step argument is -1. A simple example:

>>> l = [1, 2, 3]
>>> l[::-1]
[3, 2, 1]
>>> l[:len(l):-1]
[]

This indicates that in the case of a step argument being passed, an omitted end value is not equivalent to explicitly passing the size of the object being sliced.

Perhaps this is just a failure of mine reading the documentation, but I'd like to understand why my above example seems to contradict the Python documentation about omitting end values in slices, and ideally where this different is documented.

* 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.
2
pythonslice
l[len(l)::-1] is [3, 2, 1]. The beginning is the end when you go backwards. - Dan D.
the stop is now set at the third index, when the string is reversed by the step -1 the stop is immediately reached. That is why it returns a empty list - Sven Hakvoort
The documentation excerpt is incorrect. It only applies when the step is positive. In fact, there is no integer value that corresponds to the default for the stop index when step is -1, since it would logically be -1, except that has a special meaning to Python. You have to use None to get the desired index. It's a flaw in the slice notation. - Tom Karzes

2 Answers

6
votes

The documentation you're referencing is the tutorial, which gives only an informal overview of Python syntax and semantics. It doesn't explain all the details. You'll note that the tutorial page you linked to doesn't even discuss negative indices.

The actual documentation is given in the library reference under sequence types. Although it is a bit terse and not easy to understand on a first read, it does clarify that for a slice [i:j:k]:

If i or j are omitted or None, they become “end” values (which end depends on the sign of k).

2
votes

l[::-1] is the same thing as l.__getitem__(slice(None, None, -1)). Since the start and the stop are both None, the list will be traversed from one end to the other. The step argument determines the direction as well as the step.