from plum import dispatch

class Dog: pass
class Cat: pass

@dispatch
def greet(a: Cat, b: Dog):
    return "Hiss!"

@dispatch
def greet(a: Dog, b: Cat):
    return "Grrrr..."


# Let's try it out
cat, dog = Cat(), Dog()


print(greet(cat, dog))  # "Hiss!"
print(greet(dog, cat))  # Grrrr...