11.4 Overloading operators

Learning objectives

By the end of this section you should be able to

Identify magic methods and describe their purpose.
Develop overloaded arithmetic and comparison operators for user-defined classes.

Magic methods and customizing

Magic methods are special methods that perform actions for users, typically out of view of users. Magic methods are also called dunder methods, since the methods must start and end with double underscores (__). Ex: __init__ is a magic method used alongside __new__ to create a new instance and initialize attributes with a simple line like eng = Engineer. A programmer can explicitly define a magic method in a user-defined class to customize the method's behavior.

Overloading arithmetic operators

Operator overloading refers to customizing the function of a built-in operator. Arithmetic operators are commonly overloaded to allow for easy changes to instances of user-defined classes.

**Table 11.1** Arithmetic operators and magic methods.
Arithmetic operator (Operation)	Magic method
+ (Addition)	`__add__(self, other)`
- (Subtraction)	`__sub__(self, other)`
* (Multiplication)	`__mul__(self, other)`
/ (Division)	`__truediv__(self, other)`
% (Modulo)	`__mod__(self, other)`
** (Power)	`__pow__(self, other)`

Overloading comparison operators

Comparison operators can also be overloaded like arithmetic operators.

**Table 11.2** Comparison operators and magic methods.
Comparison operator (Operation)	Magic method
< (Less than)	`__lt__(self, other)`
> (Greater than)	`__gt__(self, other)`
<= (Less than or equal to)	`__le__(self, other)`
>= (Greater than or equal to)	`__ge__(self, other)`
== (Equal)	`__eq__(self, other)`
!= (Not equal)	`__ne__(self, other)`

Example 1 Overloading comparison operators for the Account class

**Table 11.3**
Code	Output
class Account: def __init__(self, name="", amount=0): self.name = name self.amount = amount def __str__(self): return f"{self.name}: ${self.amount}" def __lt__(self, other): return self.amount < other.amount def __gt__(self, other): return self.amount > other.amount def __eq__(self, other): return self.amount == other.amount acct_a = Account("Ashe", 6492) acct_b = Account("Bevins", 5210) print(acct_a < acct_b) print(acct_a > acct_b) acct_a.amount = 5210 print(acct_a == acct_b)	`False` `True` `True`

Note

Combining exercise logs

The ExerciseLog class has two instance attributes: e_type, the type of exercise, and duration, the time spent exercising.

Overload the + operator to combine two ExerciseLogs such that:

If the exercise types are different, combine them with " and " in between. Else, use the same type and don't duplicate.
Add durations together.

Given input:


    walk
    5
    run
    30

The output is:


    walk and run: 35 minutes

class ExerciseLog:
    def __init__(self, e_type="", duration=0):
        self.e_type = e_type
        self.duration = duration

    def __str__(self):
        return f"{self.e_type}: {self.duration} minutes"

exercise1 = input()
duration1 = int(input())
log1 = ExerciseLog(exercise1, duration1)
exercise2 = input()
duration2 = int(input())
log2 = ExerciseLog(exercise2, duration2)

log_combined = log1 + log2
print(log_combined)

Note

Expanding the Account class

Using isinstance allows the programmer to define different behaviors depending on an object's type. The first parameter is the object, and the second parameter is the type or class. Ex: isinstance(my_var, int) returns True if my_var is an integer.

Expand the existing Account example so that the addition operator can also be used to add an integer value to an Account's amount attribute.

class Account:    
    def __init__(self, name="", amount=0): 
        self.name = name
        self.amount = amount    

    def __str__(self):
        return f"{self.name}: ${self.amount}"

    def __add__(self, other):
        # Add code to allow adding int values too
        return Account(self.name + ", " + other.name,
                       self.amount + other.amount)

name_a = input()
amt_a = int(input())
acct_a = Account(name_a,  amt_a)
name_b = input()
amt_b = int(input())
acct_b = Account(name_b, amt_b)

acct_ab = acct_a + acct_b # Combine accounts
print(acct_ab)

fund = int(input())
acct_ab = acct_ab + fund
print(acct_ab)