Method Resolution Order

Part of the Object Oriented Programming section of Coddy's Python journey — lesson 21 of 64.

Method Resolution Order (MRO) is the sequence Python uses to look for methods when multiple classes have the same method name.

Here is an example with multiple classes having the same method:

class A:
    def method(self):
        return "Method from A"

class B:
    def method(self):
        return "Method from B"

class C(A, B):
    pass

Check the MRO using __mro__:

print(C.__mro__)

Output:

(<class '__main__.C'>, <class '__main__.A'>, <class '__main__.B'>, <class 'object'>)

The __mro__ attribute is a tuple that contains all classes in the resolution order. The first element (__mro__[0]) is always the class itself (C), followed by parent classes from left to right (A, then B), and finally the built-in object class.

Create an object and call the method:

c = C()
print(c.method())

Output:

Method from A

Python found the method in class A first (at index 1 in the MRO), so it used that one.

Change the inheritance order to see the difference:

class D(B, A):  # B comes before A now
    pass

print(D.__mro__)
d = D()
print(d.method())

Output:

(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.A'>, <class 'object'>)
Method from B

Now Python finds B's method first because B comes before A in the inheritance list. You can access specific classes in the MRO using indexing - D.__mro__[0] is D itself, D.__mro__[1] is B, and D.__mro__[2] is A.

You can also use the mro() method:

print(C.mro())

This gives the same result as __mro__ but as a list.

Key Point: Python searches for methods in MRO order - the class itself first (index 0), then parent classes from left to right as listed in the class definition (indices 1, 2, etc.). The first method found gets used.

Challenge

Medium

In this challenge, you'll implement a class hierarchy.

You'll need to edit these files:

device.py: Implement the base Device class
screen.py: Implement Screen class that inherits from Device
computer.py: Implement Computer class that inherits from Device
laptop.py: Implement Laptop class with multiple inheritance
driver.py - Implement test cases that will verify your implementation works correctly

Each file contains detailed TODO comments that will guide you step-by-step through the implementation.

Cheat sheet

Method Resolution Order (MRO) is the sequence Python uses to look for methods when multiple classes have the same method name.

Check MRO using __mro__ or mro():

class A:
    def method(self):
        return "Method from A"

class B:
    def method(self):
        return "Method from B"

class C(A, B):
    pass

print(C.__mro__)
print(C.mro())  # Same result as a list

Python searches for methods in MRO order - the class itself first, then parent classes from left to right as listed in the class definition:

c = C()
print(c.method())  # "Method from A" - A comes before B

class D(B, A):  # B comes before A now
    pass

d = D()
print(d.method())  # "Method from B" - B comes before A

Try it yourself

# TODO: Import all required classes
# from device import Device
# from screen import Screen
# from computer import Computer
# from laptop import Laptop

def explain_mro(class_name):
    # TODO: Print the MRO (Method Resolution Order) for the given class
    # TODO: Format: "MRO for [class name]:" (note: NO space after the colon)
    # TODO: Use class_name.__name__ to get the name of the class
    # TODO: Use class_name.__mro__ to get the MRO tuple
    # TODO: Print each class name in the MRO on separate lines
    
    # TODO: Create an instance of the class
    # TODO: Call power_on() on the instance and store the result
    # TODO: Print: "Power on result: [result]"
    # TODO: Print an empty line for better readability
    pass


# Test your code
# TODO: Call explain_mro() with each class: Device, Screen, Computer, and Laptop