I just came across this idiom in some open-source Python, and I choked on my drink.
Rather than:
if isUp:
return "Up"
else:
return "Down"
or even:
return "Up" if isUp else "Down"
the code read:
return isUp and "Up" or "Down"
I can see this is the same result, but is this a typical idiom in Python? If so, is it some performance hack that runs fast? Or is it just a once-off that needs a code review?
The "a and b or c" idiom was the canonical way to express the ternary arithmetic if in Python, before PEP 308 was written and implemented. This idiom fails the "b" answer is false itself; to support the general case, you could write
return (a and [b] or [c])[0]
An alternative way of spelling it was
return (b,c)[not a]
which, with the introduction of the bool type, could be rewritten as
return (c,b)[bool(a)]
(in case it isn't clear: the conversion to bool, and the not operator, is necessary if a is not known to be bool already)
Today, the conditional expression syntax should be used if the thing must be an expression; else I recommend to use the if statement.
You should read Using the and-or trick (section 4.6.1) of Dive Into Python by Mark Pilgrim. It turns out that the and-or trick has major pitfalls you should be aware of.
That code is a big fugly and clever for my tastes, but I suppose there's not anything wrong with it per se. I think this is really just a case of "make it all fit in one line" syndrome.
I personally would have opted for the first form though.
No, it is not.
I had a somehow similar question the other day.
if the construct
val if cond else alt
Was not very welcome ( at least by the SO community ) and the preferred one was:
if cond:
val
else:
alt
You can get your own conclusion. :)
Yikes. Not readable at all. For me pythonic means easy to read.
return isUp and "Up" or "Down"
Sounds something you would do in perl.
