Sometimes it is useful for a function to modify the objects it gets as parameters. In that case, the changes are visible to the caller. increment, which adds a given number of seconds to a Time object, can be written naturally as modifier.

def increment(self,seconds):        self.second += seconds        if (self.second>=60):            self.second-=60            self.minute+=1        if(self.minute>=60):            self.minute-=60            self.hour+=1

Is this function correct? What happens if the parameter seconds is much greater than sixty? In that case, it is not enough to carry once; we have to keep doing it until time.second is less than sixty. One solution is to replace the if statements with while statements. That would make the function correct, but not very efficient.

Write a correct version of increment that doesn’t contain any loops

def increment(self,seconds):        self.second += seconds        temp = int(self.second / 60)        if (temp>=1):            self.second-=60*temp            self.minute+=temp        temp = int(self.minute / 60)        if(temp >=1):            self.minute-=60*temp            self.hour+=temp        temp = int(self.hour / 24)        if(temp>=1):            self.hour-= 24*temp

Anything that can be done with modifiers can also be done with pure functions. In fact, some programming languages only allow pure functions. There is some evidence that programs that use pure functions are faster to develop and less error-prone than programs that use modifiers. But modifiers are convenient at times, and functional programs tend to be less efficient.

from Thinking in Python

Modifiers