This is a way prettyprint could have been written without branching:

    def prettyprint(a, b):
        fmt_string = {True:'%d is less than %d',False:'%d is higher than %d'}[a < b]
        return fmt_string % (a, b)

Or even (and this is cheating by using the int constructor):

    def prettyprint(a, b):
        fmt_string = '%d ' + ['is higher than', 'is less than'][int(a < b)] ' %d'
        return fmt_string % (a, b)

I think a good way to get acquainted with FP is to replace conditionals with lookup tables and lambda functions. But it is easy to overdo it, sometimes plain old if-statements are the most readable alternative.