Classes and OOP I

Instance Methods

You have called methods on strings and lists many times: .upper(), .append(). In this lesson you will learn to define your own methods inside a class.

You have been calling methods for a long time. Every .upper() on a string and every .append() on a list is a method call. Now you will define your own.

A method is a function defined inside a class. Its first parameter is always self:

class Dog:
	def __init__(self, name):
		self.name = name
 
	def bark(self):
		return "Woof!"

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Call a method on an instance using dot notation, just like you call "hello".upper():

class Dog:
	def __init__(self, name):
		self.name = name
 
	def bark(self):
		return "Woof!"
 
rex = Dog("Rex")
print(rex.bark())

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Output

What will be the output?

class Cat:
	def __init__(self, name):
		self.name = name
 
	def speak(self):
		return "Meow"
 
c = Cat("Luna")
print(c.speak())

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Methods can read the instance attributes through self. That is what makes them powerful.

This method uses self.radius to compute a value:

class Circle:
	def __init__(self, radius):
		self.radius = radius
 
	def diameter(self):
		return self.radius * 2
 
c = Circle(5)
print(c.diameter())

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Output

Methods can also change attributes. This is how objects keep track of their own state over time.

Each call to increment() updates the count:

class Counter:
	def __init__(self):
		self.count = 0
 
	def increment(self):
		self.count += 1
 
c = Counter()
c.increment()
c.increment()
print(c.count)

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Output

What will be the output?

class Counter:
	def __init__(self):
		self.count = 0
 
	def increment(self):
		self.count += 1
 
c = Counter()
c.increment()
c.increment()
c.increment()
print(c.count)

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Methods can take extra parameters beyond self. Think of them like regular functions with bonus access to the object.

The deposit method takes an amount parameter:

class Wallet:
	def __init__(self, cash):
		self.cash = cash
 
	def deposit(self, amount):
		self.cash += amount
 
w = Wallet(50)
w.deposit(30)
print(w.cash)

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Output

What will be the output?

class Wallet:
	def __init__(self, cash):
		self.cash = cash
 
	def spend(self, price):
		self.cash -= price
 
w = Wallet(100)
w.spend(25)
w.spend(10)
print(w.cash)

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A method can return a boolean. Useful for checking conditions on the object:

class Wallet:
	def __init__(self, cash):
		self.cash = cash
 
	def can_afford(self, price):
		return self.cash >= price
 
w = Wallet(50)
print(w.can_afford(30))
print(w.can_afford(80))

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Output

What will be the output?

class Box:
	def __init__(self, items):
		self.items = items
 
	def is_empty(self):
		return len(self.items) == 0
 
b = Box([])
print(b.is_empty())

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What will be the output?

class Greeter:
	def __init__(self, greeting):
		self.greeting = greeting
 
	def greet(self, name):
		return self.greeting + ", " + name
 
g = Greeter("Hi")
print(g.greet("Bob"))

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What will be the output?

class Score:
	def __init__(self):
		self.points = 0
 
	def add(self, n):
		self.points += n
		return self.points
 
s = Score()
result = s.add(10)
result = s.add(5)
print(result)

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