Lionel, Select is equivalent to if condition1: choice1 elsif condition2: choice2 else: default In your second example, if condition1 is false , then condition2 is always true: ( ! t>90 => t<90 => t<300). Same thing if you reverse the order of the conditions, of course. I find the base functions greater, equal, less... quite useful: select([greater(t,90)*less(t,300)],[t,]) is a good start.