2.5 Designing complex if...elif chains

In most cases, our scripts will involve a number of choices. Sometimes the choices are simple, and we can judge the quality of the design with a glance at the code. In other cases, the choices are more complicated, and it’s not easy to determine whether or not our if statements are designed properly to handle all of the conditions.

In the simplest case, we have one condition, C, and its inverse, ¬C. These are the two conditions for an if...else statement. One condition, C, is stated in the if clause; the inversion condition, ¬C, is implied in the else clause.

This follows the Law of the Excluded Middle: we’re claiming there’s no missing alternative between the two conditions, C and ¬C. For a complex condition, though, this can be difficult to visualize.

If we have something like:

 
 
if weather == Weather.RAIN and plan == Plan.GO_OUT: 
 
    ...