dog.py

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age
        pass

    def bark(self):
        print("bark bark!")

    def doginfo(self):
        print(self.name + " is " + str(self.age) + " year(s) old.")
    
    def birthday(self):
        self.age += 1

    def set_buddy(self, buddy):
        self.buddy = buddy
        buddy.buddy = self

ozzy = Dog("Ozzy", 2)
skippy = Dog("Skippy", 12)
filou = Dog("Filou", 8)

ozzy.doginfo()
skippy.doginfo()
filou.doginfo()

ozzy.age = 3
ozzy.doginfo()

ozzy.birthday()
ozzy.doginfo()

ozzy.bark()

ozzy.set_buddy(filou)

print(ozzy.buddy.name)
print(ozzy.buddy.age)

print(filou.buddy.name)
print(filou.buddy.age)

ozzy.buddy.doginfo()