Understanding pythons reverse slice ( [::-1] )

问题

I always thought that omitting arguments in the python slice operation would result into:

• start = 0
• end = len(lst)
• step = 1

That holds true if the step is positive, but as soon as the step is negative, like in the "reverse slice" [::-1], omitting start/end results in:

• start = len(lst)-1
• end = None

Is this a special case, or am I missing something?

回答1:

The default is always None; it is up to the type to determine how to handle None for any of the 3 values. The list object is simply passed a slice(None, None, -1) object in this case.

See footnote 5 to the operations table in the sequence types documentation for how Python's default sequence types (including list objects) interpret these:

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

So the defaults are dependent on the sign of the step value; if negative the ends are reversed. For [::-1] the end values are len(s) - 1 and -1 (absolute, not relative to the end), respectively, because the step is negative.

来源：https://stackoverflow.com/questions/36238612/understanding-pythons-reverse-slice-1