Method Resolution Order (MRO) in Python

Introduction

Method Resolution Order (MRO) decides the order in which Python looks for methods in classes when you use inheritance. It helps Python know which method to run when multiple classes have methods with the same name.

When you have a class that inherits from more than one parent class.

When you want to understand which method will run if multiple parents have the same method name.

When debugging why a certain method is called in a complex class hierarchy.

When designing classes that share behavior and want to control method lookup order.

Syntax

Python

class Child(Parent1, Parent2):
    pass

# To see MRO:
print(Child.__mro__)

The MRO is a tuple showing the order Python checks classes for methods.

Python uses the C3 linearization algorithm to create the MRO.

Examples

Shows MRO for class C which inherits from B, which inherits from A.

Python

class A:
    def greet(self):
        print('Hello from A')

class B(A):
    def greet(self):
        print('Hello from B')

class C(B):
    pass

print(C.__mro__)

Shows MRO for class Z with multiple inheritance from X and Y.

Python

class X:
    def greet(self):
        print('Hello from X')

class Y:
    def greet(self):
        print('Hello from Y')

class Z(X, Y):
    pass

print(Z.__mro__)

Sample Program

Class D inherits from B and C. We print the MRO to see the order Python looks for methods. Then we call greet() to see which method runs.

Python

class A:
    def greet(self):
        print('Hello from A')

class B(A):
    def greet(self):
        print('Hello from B')

class C(A):
    def greet(self):
        print('Hello from C')

class D(B, C):
    pass

print('MRO of D:', D.__mro__)

obj = D()
obj.greet()

Important Notes

The MRO helps avoid confusion when multiple parents have the same method.

The order in the class definition (e.g., class D(B, C)) affects the MRO.

You can always check MRO using ClassName.__mro__ or help(ClassName).

Summary

MRO decides the order Python looks for methods in inheritance.

It is important for multiple inheritance to know which method runs.

You can check MRO with ClassName.__mro__ to understand method lookup.

Practice

1. What does Method Resolution Order (MRO) in Python determine?

easy

A. The order Python compiles code

B. The order Python executes loops

C. The order Python looks for methods in inheritance

D. The order Python imports modules

Solution

1. Step 1: Understand MRO purpose

   MRO defines the sequence Python follows to find methods in classes with inheritance.

2. Step 2: Compare options

   Only The order Python looks for methods in inheritance correctly describes MRO's role in method lookup order.

3. Final Answer:

   The order Python looks for methods in inheritance -> Option C

4. Quick Check:

   MRO = method lookup order [OK]

Hint: MRO is about method search order in inheritance [OK]

Common Mistakes:

• Confusing MRO with loop or import order
• Thinking MRO controls code compilation
• Mixing MRO with unrelated Python features

2. Which of the following is the correct way to check the MRO of a class MyClass in Python?

easy

A. print(MyClass.__mro__)

B. print(MyClass.get_mro())

C. print(MyClass.MRO())

D. print(MyClass.mro)

Solution

1. Step 1: Recall MRO access methods

   Python provides __mro__ attribute and mro() method to check MRO.

2. Step 2: Identify correct syntax

   MyClass.__mro__ is a tuple showing MRO; MyClass.mro() is a method returning a list. print(MyClass.__mro__) uses __mro__ correctly with print.

3. Final Answer:

   print(MyClass.__mro__) -> Option A

4. Quick Check:

   Use __mro__ attribute to check MRO [OK]

Hint: Use ClassName.__mro__ to see MRO tuple [OK]

Common Mistakes:

• Using non-existent get_mro() method
• Forgetting parentheses for mro() method
• Trying to print mro without calling it

3. What will be the output of the following code?

class A:
    def greet(self):
        return 'Hello from A'

class B(A):
    def greet(self):
        return 'Hello from B'

class C(A):
    def greet(self):
        return 'Hello from C'

class D(B, C):
    pass

print(D().greet())

medium

A. 'Hello from B'

B. 'Hello from A'

C. 'Hello from C'

D. Error: Ambiguous method

Solution

1. Step 1: Determine MRO of class D

   Class D inherits from B and C. Python uses C3 linearization: D > B > C > A.

2. Step 2: Find first greet method in MRO

   Method greet is found first in B, so D().greet() calls B's greet method.

3. Final Answer:

   'Hello from B' -> Option A

4. Quick Check:

   MRO order picks B's greet first [OK]

Hint: MRO checks parents left to right, first method wins [OK]

Common Mistakes:

• Assuming C's greet is called instead of B's
• Thinking A's greet is called directly
• Expecting an error due to multiple inheritance

4. Consider the following code snippet. What is the error and how to fix it?

class X:
    def method(self):
        return 'X'

class Y:
    def method(self):
        return 'Y'

class Z(X, Y):
    def method(self):
        return super().method()

print(Z().method())

medium

A. Error: super() call is ambiguous; fix by specifying class and self

B. Output: 'X' (no error)

C. Output: 'Y' (no error)

D. Error: Missing parentheses in print statement

Solution

1. Step 1: Analyze super() in Z.method()

   super() calls next method in MRO after Z, which is X.method().

2. Step 2: Check output of X.method()

   X.method() returns 'X', so print outputs 'X' with no error.

3. Final Answer:

   Output: 'X' (no error) -> Option B

4. Quick Check:

   super() calls next in MRO, here X.method() [OK]

Hint: super() calls next method in MRO automatically [OK]

Common Mistakes:

• Thinking super() needs explicit class and self
• Expecting output 'Y' instead of 'X'
• Assuming syntax error in print statement

5. Given the classes below, what is the MRO of class F?

class A: pass
class B(A): pass
class C(A): pass
class D(B, C): pass
class E(C, B): pass
class F(D, E): pass

hard

A. (F, E, D, B, C, A, object)

B. (F, D, E, B, C, A, object)

C. (F, D, B, C, E, C, B, A, object)

D. TypeError due to inconsistent MRO

Solution

1. Step 1: Understand MRO consistency rules

   Python requires MRO to be consistent and follow C3 linearization rules.

2. Step 2: Check classes D and E inheritance

   D inherits B then C; E inherits C then B. This creates conflicting order for F inheriting D and E.

3. Step 3: Result of conflict

   Python raises TypeError for class F due to inconsistent MRO from conflicting parent orders.

4. Final Answer:

   TypeError due to inconsistent MRO -> Option D

5. Quick Check:

   Conflicting parent order causes TypeError [OK]

Hint: Conflicting parent order causes MRO TypeError [OK]

Common Mistakes:

• Assuming Python picks one MRO silently
• Ignoring C3 linearization rules
• Trying to list MRO despite conflict