Question

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?

Answer 1

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.