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'example'[999:9999] result in error? Since
'example' does, what is the motivation behind it?
From this behavior I can assume that
'example' is, essentially/internally, not the same as
'example'[3:4], even though both result in the same
'example' are fundamentally different, and slicing outside the bounds of a sequence (at least for built-ins) doesn’t cause an error.
It might be surprising at first, but it makes sense when you think about it. Indexing returns a single item, but slicing returns a subsequence of items. So when you try to index a nonexistent value, there’s nothing to return. But when you slice a sequence outside of bounds, you can still return an empty sequence.
Part of what’s confusing here is that strings behave a little differently from lists. Look what happens when you do the same thing to a list:
0, 1, 2, 3, 4, 5] 3 [0, 1, 2, 3, 4, 5][3:4] [
Here the difference is obvious. In the case of strings, the results appear to be identical because in Python, there’s no such thing as an individual character outside of a string. A single character is just a 1-character string.
(For the exact semantics of slicing outside the range of a sequence, see mgilson’s answer.)
For the sake of adding an answer that points to a robust section in the documentation:
Given a slice expression like
The slice of s from i to j with step k is defined as the sequence of items with index
x = i + n*ksuch that
0 <= n < (j-i)/k. In other words, the indices are
i+3*kand so on, stopping when j is reached (but never including j). When k is positive, i and j are reduced to
len(s)if they are greater
if you write
s[999:9999], python is returning
len(s) < 999 and your step is positive (
1 — the default).
Slicing is not bounds-checked by the built-in types. And although both of your examples appear to have the same result, they work differently; try them with a list instead.