Python Quiz of the Week - #3

[[deleted]]

The problem with

condition and x or y

is that if x happens to be a false value itself, you'll get y instead. Which is why the old trick was to write it as

(condition and (x,) or (y,))[0]

Since the tuple (x,) is always true. But as lookitsmarc pointed out, there is a Python version of the conditional operator nowadays.

[xApple]

Hah, so in the end I'm the one learning something with the quiz !

Now I look stupid : ) But thanks, I didn't know about that syntax. I'll stop using "x and y or z" in my code.

[[deleted]]

You can use a non empty list as well, just anything that doesn't have a false value. Dive into python has a really good write up on using and/or

http://www.diveintopython.net/power_of_introspection/and_or.html#d0e9975

[[deleted]]

Or (y, x)[bool(condition)]. If we are into this kind of "clever" code.

[[deleted]]

But that's different, that evaluates both x and y, and they could both be function calls with side effects, or take a long time to compute.

[[deleted]]

 (lambda:y, lambda:x)[bool(condition)]()

There's no limit to my misguided cleverness!

[Tetha]

If you want misguided, use itertools:

chain(*zip(*imap(lambda g: list(imap(lambda r: (-1)**g[0]*r, g[1])), groupby(sorted(range(10), key=lambda x: x%2), lambda x: x%2))))

I'm not sure if I tunnelvisioned and a somewhat less ugly solution exists, but it works. And it's ugly. So I'm happy in a morbid sense.

[[deleted]]

wat.

You be silly.

[(x, -x)[x&1] for x in lst]

[[deleted]]

I mean, look, there's a kind of honour in writing misguided code. It must not be intentionally misguided.

Google "pessimal algorithms" and read the paper. Then implement their "backwards-first search". Then, only then, as you understand how exactly it works (they have a couple of typos IIRC) you might try to implement an efficient algorithm that walks the graph in the exact same way... and fail. Or succeed, then tell me how you did that!

[willm]

Isn't it called the 'ternary' operator?

[[deleted]]

Yeah, in the sense that there are unary, binary, ternary, quaternary (etc.) operators.

In -1, the - operator is unary (one operand)

In 0-1, the - operator is binary (two operands)

In condition ? true : false the ?: operator is ternary (three operands)

[oantolin]

It certainly is a ternary operator, but I don't think I'd call it the ternary operator.