1. I'm unfamiliar with the term 'unpacking'. Is it any different from pattern matching in, say, Haskell (but perhaps not as feature-rich)?
2. Aren't slices pretty much a staple in Python? I didn't think using them was considered a 'trick'.
unpacking < destructuring < pattern matching < first-class patterns
That's the haskell extension.
To see a very nice use of them, check out this paper (pdf) http://strictlypositive.org/CJ.pdf
I spoke too eagerly—the new feature is just named and namespaced patterns. It's a bit of a bump in power, but it's not fully general yet.
For true(-ish) first-class patterns take a look at Prisms in the lens package or some of the other first-class pattern libraries.
a,(b,c),d = [1,[2,3],4]I think that Python's unpacking allows most, if not all, of Clojures sequence destructuring for tuples and lists. Clojure takes it a bit further, however, by applying it to all sequences. For example, you could do this:
a,b,c = "XYZ"
a,b : c as d = [1,2,3,4,5]
# a = 1
# b = 2
# c = [3, 4, 5]
# d = [1, 2, 3, 4, 5]
Where destructuring really shines, though, is that you can destructure maps (dictionaries in Python) and vectors can also be treated as maps (their keys are the indices), so you can do stuff like this:
a, {[b {:keys [c, d]} :foo}, e = [1, {foo: [2, {c: 3, d: 4}, bar: 9}, 5]
# a = 1
# b = 2
# c = 3
# d = 4
# e = 5
t = (1, 2)
a, b = t
# a == 1 and b == 2
php > $t = array(1, 2);
php > list($a, $b) = $t;
php > echo "$a, $b";
1, 2