Colon colon: just when you thought you knew Python

For a simple scripting language Python bears more then enough brain-exploding magic, like metaclasses and descriptors. But what if a simple language construct, in a simple code, just makes you stop and say “WAT?”. That’s exactly what happened to me when I saw this: x = x[::-1]. What does that mean? Let’s take a quick look.

The Docs ¶

The “official” definition how subscriptions work in Python can, with certain luck, be found in Python docs:

s[i:j:k] slice of s from i to j with step k

And a note below says:

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

So, with x[::-1] we are taking the whole x (similarly to x[:]), but with step -1 – which means, we are having x reversed.

The Examples ¶

Let’s try it:

>>> [1, 2, 3][::-1]
[3, 2, 1]
>>> ('answer', 42)[::-1]
(42, 'answer')
>>> 'foo bar'[::-1]
'rab oof'

What I also have to notice is that this construct (unlike, e.g., [reversed] builtin) preserves the type of its argument: for tuple, you get tuple, and for string, you get string. This is often convenient.

The End ¶

Well, the more you learn…