else condition in list comprehension
Dan Bishop
danb_83 at yahoo.com
Sun Jan 9 17:04:40 EST 2005
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 [sic] to be "i-2" if i%2 is not equal to 0?
z = [i + (2, -2)[i % 2] for i in range(10)]
In general, the expression "T if C is true, or F if C is false" can be
written as (F, T)[bool(C)]. (If you know that C will always be either
0 or 1, as is the case here, the "bool" is redundant.)
Unless, of course, either F or T has side effects. For a side-effect
free expression, you can use (C and [T] or [F])[0] or one of the many
other ternary operator substitutes. (Search for PEP 308.)
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