Multiple Inheritance with Multiple Base classes

Question

please help me understand the concept of multiple inheritance here in Python (I am from C# background which does not support multiple inheritance).

I am using Python 3.7.6.

In the below code, the class Apple inherits the classes ToyFruit, NaturalFruit, FakeFruit and RoboticFruit. While ToyFruit, NaturalFruit, and FakeFruit inherit Fruit base class, RoboticFruit has a different BaseClass Robot.

I notice that the RoboticFruit and Robot are not at all getting called.

class Fruit:
    def __init__(self, name):
        print("This is the Fruit __init__ function")
        self.test = "BaseClass"
        self.name = name
        print("Fruit object created")


class NaturalFruit(Fruit):
    def __init__(self, name):
        print("This is the NaturalFruit __init__ function")
        super().__init__(name)
        self.type = "Natural"
        print("This is a Natural Fruit")
        self.test = "NaturalClass"
        print("Natural Fruit object created")


class FakeFruit(Fruit):
    def __init__(self, name):
        print("This is the FakeFruit __init__ function")
        super().__init__(name)
        self.type = "Fake"
        print("This is a Fake Fruit")
        self.test = "FakeClass"
        print("Fake Fruit object created")


class ToyFruit(Fruit):
    def __init__(self, name):
        print("This is the ToyFruit __init__ function")
        super().__init__(name)
        self.type = "Toy"
        print("This is the Toy Fruit")
        self.test = "ToyClass"
        print("Toy Fruit object created")


class Robot:
    def __init__(self, name):
        print("This is the ROBOT __init__ function")
        self.test = "RobotClass"
        self.name = name
        print("Robot object created")


class RoboticFruit(Robot):
    def __init__(self, name):
        super().__init__("RoboticFruit")
        print("Robotic Fruit")


class Apple(ToyFruit, NaturalFruit, FakeFruit, RoboticFruit):
    def __init__(self):
        super().__init__("Apple")
        print("Apple object created")


apple = Apple()
# print(apple.name)
print(apple.test)

OUTPUT:-

This is the ToyFruit __init__ function
This is the NaturalFruit __init__ function
This is the FakeFruit __init__ function
This is the Fruit __init__ function
Fruit object created
This is a Fake Fruit
Fake Fruit object created
This is a Natural Fruit
Natural Fruit object created
This is the Toy Fruit
Toy Fruit object created
Apple object created
ToyClass

If I swap the order to

class Apple(RoboticFruit, ToyFruit, NaturalFruit, FakeFruit):

Then the ToyFruit, NaturalFruit, FakeFruit and Fruit __init__ methods are not called at all. I am not understanding why the RoboticFruit class constructors are getting skipped.

Answer 1

I'm not sure that it returns the result you want, but it could be an option to replace super() calls to __init__ with hardcoded calls:

class Fruit:
    def __init__(self, name):
        print("This is the Fruit __init__ function")
        self.test = "BaseClass"
        self.name = name
        print("Fruit object created")


class NaturalFruit(Fruit):
    def __init__(self, name):
        print("This is the NaturalFruit __init__ function")
        Fruit.__init__(self, name)
        self.type = "Natural"
        print("This is a Natural Fruit")
        self.test = "NaturalClass"
        print("Natural Fruit object created")


class FakeFruit(Fruit):
    def __init__(self, name):
        print("This is the FakeFruit __init__ function")
        Fruit.__init__(self, name)
        self.type = "Fake"
        print("This is a Fake Fruit")
        self.test = "FakeClass"
        print("Fake Fruit object created")


class ToyFruit(Fruit):
    def __init__(self, name):
        print("This is the ToyFruit __init__ function")
        Fruit.__init__(self, name)
        self.type = "Toy"
        print("This is the Toy Fruit")
        self.test = "ToyClass"
        print("Toy Fruit object created")


class Robot:
    def __init__(self, name):
        print("This is the ROBOT __init__ function")
        self.test = "RobotClass"
        self.name = name
        print("Robot object created")


class RoboticFruit(Robot):
    def __init__(self, name):
        Robot.__init__(self, "RoboticFruit")
        print("Robotic Fruit")


class Apple(ToyFruit, NaturalFruit, FakeFruit, RoboticFruit):
    def __init__(self):
        ToyFruit.__init__(self, "Apple")
        NaturalFruit.__init__(self, "Apple")
        FakeFruit.__init__(self, "Apple")
        RoboticFruit.__init__(self, "Apple")
        print("Apple object created")


apple = Apple()
#print(apple.name)
print(apple.test)

# output:
# This is the ToyFruit __init__ function
# This is the Fruit __init__ function
# Fruit object created
# This is the Toy Fruit
# Toy Fruit object created
# This is the NaturalFruit __init__ function
# This is the Fruit __init__ function
# Fruit object created
# This is a Natural Fruit
# Natural Fruit object created
# This is the FakeFruit __init__ function
# This is the Fruit __init__ function
# Fruit object created
# This is a Fake Fruit
# Fake Fruit object created
# This is the ROBOT __init__ function
# Robot object created
# Robotic Fruit
# Apple object created
# RobotClass

Answer 2

In case of multiple inheritance, super() delegates to the next object in the Method Resolution Order (MRO). We can see the MRO of the class Apple:

print(Apple.__mro__)

# output:
(
<class '__main__.Apple'>, 
<class '__main__.ToyFruit'>, 
<class '__main__.NaturalFruit'>, 
<class '__main__.FakeFruit'>, 
<class '__main__.Fruit'>, 
<class '__main__.RoboticFruit'>, 
<class '__main__.Robot'>, 
<class 'object'>
)

So I suppose that RoboticFruit and Robot are not called because there is no call like super().__init__(name) in class Fruit, which is the previous one to RoboticFruit in that order (MRO). If you add a call to super() in Fruit, it should work correctly:

class Fruit:
    def __init__(self, name):
        print("This is the Fruit __init__ function")
        super().__init__(name)
        self.test = "BaseClass"
        self.name = name
        print("Fruit object created")