Instance methods

Source: Dev.to

Instance methods

After learning about classes, let’s learn about methods.

class Time:
    def __init__(self):
        self.hours = 0
        self.minutes = 0

    def print_time(self):
        print(f'Hours: {self.hours}', end=' ')
        print(f'Minutes: {self.minutes}')


time1 = Time()
time1.hours = 3
time1.minutes = 35
time1.print_time()

In the example above, print_time is an instance method—a function defined inside a class that operates on a particular instance.
The __init__ method is a special instance method that runs automatically when a new object is created, initializing its attributes.

A common mistake for beginners is to omit the self argument as the first parameter of a method. Because Python automatically passes the instance reference as the first argument, forgetting self leads to a “too many arguments” error.

Common error: forgetting self

class Employee:
    def __init__(self):
        self.wage = 0
        self.hours_worked = 0

    def calculate_pay(self):  # `self` must be included
        return self.wage * self.hours_worked


alice = Employee()
alice.wage = 6.25
alice.hours_worked = 15
print(f'Alice earned {alice.calculate_pay():.2f}')

If the self parameter is omitted (e.g., def calculate_pay():), calling alice.calculate_pay() will raise an error because Python tries to pass the instance automatically, resulting in a mismatch of arguments. Adding self resolves the issue.