Arithmetic operators

The arithmetic operators can be overloaded too. a + b calls a.__add__(b), and whatever that method returns is the result. These are the methods:

Here's a class for durations, such as the length of a song. Adding two durations gives a new duration:

class Duration:
    def __init__(self, minutes, seconds):
        self.minutes = minutes
        self.seconds = seconds

    def __str__(self):
        return f"{self.minutes} min {self.seconds} s"

    def total_seconds(self):
        return self.minutes * 60 + self.seconds

    def __add__(self, other):
        total = self.total_seconds() + other.total_seconds()
        return Duration(total // 60, total % 60)


intro = Duration(3, 40)
solo = Duration(4, 35)
both = intro + solo
print(both)
print(intro)
print(solo)

8 min 15 s
3 min 40 s
4 min 35 s

__add__ returns a new object and leaves both of the originals as they were. That's what + does with numbers too: 2 + 3 doesn't change the 2. If __add__ changed self instead, both = intro + solo would quietly change intro as well, and that's a surprise nobody wants.

The other operand can be something else

The two sides of an operator don't have to be of the same class. Multiplying a duration by a whole number makes sense: a song played three times. A difference that would be negative doesn't make sense, so __sub__ raises a ValueError, as a class does when it's given a value it can't accept:

class Duration:
    def __init__(self, minutes, seconds):
        self.minutes = minutes
        self.seconds = seconds

    def __str__(self):
        return f"{self.minutes} min {self.seconds} s"

    def total_seconds(self):
        return self.minutes * 60 + self.seconds

    def __sub__(self, other):
        total = self.total_seconds() - other.total_seconds()
        if total < 0:
            raise ValueError("a duration can't be negative")
        return Duration(total // 60, total % 60)

    def __mul__(self, times):
        total = self.total_seconds() * times
        return Duration(total // 60, total % 60)


intro = Duration(3, 40)
solo = Duration(4, 35)
print(intro * 3)
print(solo - intro)
print(intro - solo)

11 min 0 s
0 min 55 s

Traceback (most recent call last):
  File "main.py", line 27, in <module>
    print(intro - solo)
          ~~~~~~^~~~~~
  File "main.py", line 15, in __sub__
    raise ValueError("a duration can't be negative")
ValueError: a duration can't be negative

In intro * 3, self is intro and times is 3.

+= comes for free

a += b is short for a = a + b, so it works as soon as __add__ does. It makes a new object and puts it in the variable. That's handy for a total:

total = Duration(0, 0)
for song in album:
    total += song

As with comparisons, write only the operators that mean something for your class. Adding two durations makes sense; dividing a duration by another duration, maybe not.