Diamond problem in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method call in diamond inheritance

Consider the following Python code using diamond inheritance. What is the output when obj.greet() is called?

Python

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

obj = D()
print(obj.greet())

A "Hello from C"

B "Hello from A"

C "Hello from B"

D TypeError

💡 Hint

Remember Python uses Method Resolution Order (MRO) to decide which method to call.

Class D inherits from B and C. Python checks B first in the MRO, so it uses B's greet method.

❓ Predict Output

intermediate

Understanding MRO list in diamond inheritance

What is the output of print(D.__mro__) for the following classes?

Python

class A:
    pass

class B(A):
    pass

class C(A):
    pass

class D(B, C):
    pass

print(D.__mro__)

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

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

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

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

💡 Hint

MRO follows the order of inheritance from left to right and then up the hierarchy.

Python's MRO for class D is D, B, C, A, object because B is listed before C in D's inheritance.

❓ Predict Output

advanced

Output with super() in diamond inheritance

What will be printed when D().greet() is called in this code?

Python

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

class B(A):
    def greet(self):
        return "B says " + super().greet()

class C(A):
    def greet(self):
        return "C says " + super().greet()

class D(B, C):
    def greet(self):
        return "D says " + super().greet()

print(D().greet())

A "D says B says C says Hello from A"

B "D says C says B says Hello from A"

C "D says Hello from A"

D AttributeError

💡 Hint

super() follows the MRO chain calling next method in line.

The call chain is D.greet -> B.greet -> C.greet -> A.greet, concatenating strings in order.

❓ Predict Output

advanced

Effect of changing inheritance order on output

What is the output of D().greet() if class D inherits as class D(C, B): instead of class D(B, C): in the previous example?

Python

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

class B(A):
    def greet(self):
        return "B says " + super().greet()

class C(A):
    def greet(self):
        return "C says " + super().greet()

class D(C, B):
    def greet(self):
        return "D says " + super().greet()

print(D().greet())

A "D says C says B says Hello from A"

B "D says B says C says Hello from A"

C "D says Hello from A"

D TypeError

💡 Hint

Changing the order of inheritance changes the MRO and thus the order super() calls methods.

Now the MRO is D, C, B, A, object, so super() calls C.greet then B.greet then A.greet.

🧠 Conceptual

expert

Why Python's diamond problem solution is safe

Why does Python's method resolution order (MRO) prevent the classic diamond problem issues seen in some other languages?

A Because Python uses depth-first search to find methods, avoiding duplicates.

B Because Python uses a linearization algorithm (C3) that ensures each class appears only once in the MRO, preventing duplicate calls.

C Because Python requires explicit calls to parent methods, so no automatic method calls happen.

D Because Python does not support multiple inheritance, so diamond problem cannot occur.

💡 Hint

Think about how Python orders classes in the MRO list.

Python uses the C3 linearization algorithm to create an MRO that includes each class once, so methods are called only once, avoiding diamond problem issues.