Favorite non-python language trick?

[Joseph Garvin]

As someone who learned C first, when I came to Python everytime I read 
about a new feature it was like, "Whoa! I can do that?!" Slicing, dir(), 
getattr/setattr, the % operator, all of this was very different from C.

I'm curious -- what is everyone's favorite trick from a non-python 
language? And -- why isn't it in Python?

Here's my current candidate:

So the other day I was looking at the language Lua. In Lua, you make a 
line a comment with two dashes:

-- hey, this is a comment.

And you can do block comments with --[[ and ---]].

--[[
hey
this
is
a
big
comment
--]]

This syntax lets you do a nifty trick, where you can add or subtract a 
third dash to change whether or not code runs:

--This code won't run because it's in a comment block
--[[
print(10)
--]]

--This code will, because the first two dashes make the rest a comment, 
breaking the block
---[[
print(10)
--]]

So you can change whether or not code is commented out just by adding a 
dash. This is much nicer than in C or Python having to get rid of """ or 
/* and */. Of course, the IDE can compensate. But it's still neat :)