12.5 Inheritance

Inheritance

# Inheritance in coding is when one "child" class receives
# all of the methods and attributes of another "parent" class


class Test:
    def __init__(self):
        self.x = 0


# class Derived_Test inherits from class Test
class Derived_Test(Test):
    def __init__(self):
        Test.__init__(self)  # do Test's __init__ method
        # Test's __init__ gives Derived_Test the attribute 'x'
        self.y = 1


b = Derived_Test()

# Derived_Test now has an attribute "y", even though
# it originally didn't
print(b.x, b.y)

• It allows for you to reuse code

• It's more efficient for the programmer
• In the example above, we just did Test's __init__ method and didn't have to explicitly define an x attribute for Derived_Test

What can you inherit?

class Furniture:
	def __init__(self):
		self.built = True
		self.material = "nothing"

class Sofa(Furniture):
	def __init__(self):
		super().__init__()  # sets built to True
		self.material = "comfy stuff"  # this overrides Furniture's material
		self.comfy = True
	def destroy(self):
		self.built = False

lazy_boy_couch = Sofa()
print(lazy_boy_couch.material)  # prints "comfy stuff" instead
 # of 'nothing' since we overrode it

lazy_boy_couch.destroy()
print(lazy_boy_couch.built)  # would print False since we set it to False

One more Example

class bird:
	def __init__(self, wingspan):
		self.living = True  # all birds will be living
		self.wingspan = wingspan  # wingspan will be set to whatever is provided
	def fly(self):
		print("it's flying")
	def eat(self):
		print("it's eating")

class penguin(bird):  # penguing is a child of bird
	def __init__(self, wingspan):  # since it's a child of bird,
		# it needs to take at least as many parameters as bird

		super().__init__(wingspan)  # passes wingspan to bird that
		# way bird can complete its init (which needs a provided wingspan)

	def fly(self):
		# this is an example of overriding a method
		# instead of fly doing what bird's fly method does,
		# it will do the following instead
		print("penguins can't fly")


emperor_peng = penguin(30)  # 30 is the wingspan

emperor_peng.eat()  # we didn't define it inside penguin, but
# penguin inherited it from bird. It will print "it's eating"

emperor_peng.fly()  # we overrode it from bird, so it will
# do penguin's fly, which is printing "penguins can't fly"

print(emperor_peng.wingspan)  # will print 30. This is because
# we passed 30 to the penguin class as the wingspan. The penguin
# class then passed 30 as the wingspan to super().__init__, or bird's init
# So, bird used wingspan, which was 30, to set our penguin's wingspan to 30

print(emperor_peng.living)  # will print True since bird's init sets it to True

Practice

Food Class

• an eat method that prints "You are eating" and the name of the food

• a burn method that prints "You burned [x] calories" where [x] is the number of calories in the food

Buildings