else condition in list comprehension

[Reinhold Birkenfeld]

Matteo Dell'Amico wrote:
> Luis M. Gonzalez wrote:
>> Hi there,
>> 
>> I'd like to know if there is a way to add and else condition into a
>> list comprehension. I'm sure that I read somewhere an easy way to do
>> it, but I forgot it and now I can't find it...
>> 
>> for example:
>> z=[i+2 for i in range(10) if i%2==0]
>> what if I want i to be "i-2" if i%2 is not equal to 0?
> 
> You could use
> 
> [(i-2, i+2)[bool(i%2 == 0)] for i in range(10)]
> 
> or, in a less general but shorter way
> 
> [(i+2, i-2)[i%2] for i in range(10)]
> 
> or even
> 
> [i%2 and i-2 or i+2 for i in range(10)]

One should note that the (cond and X or Y) construct only works if X can
never produce a false value (such as 0, "", []). In this example, it is
okay, but replace 2 with 1 and you will run into trouble for i = 1.

Reinhold