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.

Checkpoint: Customizing __str__() in a user-defined class, Engineer

Concepts in Practice \(\PageIndex{1}\): Magic methods

Which of the following is a magic method?

1. add()
2. _add_()
3. __add__()

Answer

c. __add__() is a magic method, indicated by the double underscores at the start and end of the method name.

Concepts in Practice \(\PageIndex{2}\): Magic methods

Why are magic methods special?

1. can't be called by the user
2. perform internal actions
3. have fixed definitions

Answer

b. Magic methods are designed to be invoked internally to perform under-the-hood actions to simplify tasks for the user.

Concepts in Practice \(\PageIndex{3}\): Magic methods

Consider the example above, and identify the magic method(s) in the updated program.

1. __init__()
2. __str__()
3. __init__(), __str__()

Answer

c. __init__() is defined by the programmer to initialize instance attributes. __str__() is defined by the programmer to customize an Engineer instance's string representation.

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.

Checkpoint: Overloading __add__() for a user-defined class, Account

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)

Concepts in Practice \(\PageIndex{4}\): Arithmetic operator overloading

A programmer explicitly defining the modulo operator for their user-defined class is ___________ the operator.

1. overloading
2. rewriting
3. implementing

Answer

a. A programmer can overload an operator to customize the built-in operator's function.

Concepts in Practice \(\PageIndex{5}\): Arithmetic operator overloading

Given the code below, which argument maps to the parameter other in the call to Account.__add__()?

acct_a = Account("Ashe", 6492)
acct_b = Account("Bevins", 5210)

acct_ab = acct_a + acct_b

1. acct_a
2. acct_b
3. acct_ab

Answer

b. The addition operation maps the left operand, acct_a, to the parameter self and the right operand, acct_b, to the parameter other.

Concepts in Practice \(\PageIndex{6}\): Arithmetic operator overloading

Which __sub__() definition overloads the - operator for the code below to work?

class Pair:
  def __init__(self, x=0, y=0):
    self.x = x
    self.y = y
  # Define __sub__()

p1 = Pair(10, 2)
p2 = Pair(8, 4)
p3 = p1 - p2
print(p3.x, p3.y)

1.   def __sub__(self):
       return Pair(self.x - other.x, self.y - other.y)
   

2.   def __sub__(self, other):
       return self.x - other.x, self.y - other.y
   

3.   def __sub__(self, other):
       return Pair(self.x - other.x, self.y - other.y)

Answer

c. - is a binary operator, so __sub__() has two parameters. The parameters' x and y attributes are subtracted from each other, and the resulting values are made into a new Pair.

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 11.2: 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

Concepts in Practice: Comparison operator overloading

Consider the example above.

Concepts in Practice \(\PageIndex{7}\): Comparison operator overloading

How many operators are overloaded?

1. 3
2. 4
3. 5

Answer

a. <, >, and == are overloaded with __lt__(), __gt__(), and __eq__(), respectively.

Concepts in Practice \(\PageIndex{8}\): Comparison operator overloading

Which code appropriately overloads the <= operator?

1.   def __le__(other):
       return self.amount <= other.amount
   

2.   def __le__(self, other):
       return self.amount <= other.amount
   

3.   def __le__(self, other):
       return other.amount <= self.amount

Answer

b. __le__() is the magic method for the <= operator. __le__() takes two parameters for the left and right operand, and returns the correct comparison of the two amount values.

Concepts in Practice \(\PageIndex{9}\): Comparison operator overloading

Which type of value does __gt__() return?

1. account instance
2. Boolean
3. integer

Answer

b. __gt__() overloads the > operator, which makes a comparison and returns True or False .

Try It: 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

Interactive Code

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)

Input

cycle
45
run
15

Answer

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"

def __add__(self, other):
if self.e_type != other.e_type:
return ExerciseLog(f"{self.e_type} and {other.e_type}", self.duration + other.duration)
else:
return ExerciseLog(self.e_type, self.duration + other.duration)

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)

Try It: 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.

Interactive Code

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)

Input

Ashe
4959
Bevins
8374

Answer

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):
if isinstance(other, Account):
return Account(self.name + ", " + other.name,
self.amount + other.amount)
elif isinstance(other, int):
return Account(self.name, self.amount + other)

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)