Diamond problem in Python - Step-by-Step Execution

Concept Flow - Diamond problem

Class A

↓

Class B

↓

Class D

↓

Create instance of D

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Method call

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Resolve method via MRO

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Execute method

Shows how Python resolves method calls in a diamond inheritance pattern using Method Resolution Order (MRO).

Execution Sample

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

obj = D()
obj.greet()

Defines classes in a diamond shape and calls greet() on D instance to show MRO in action.

Execution Table

Step	Action	Evaluation	Result
1	Create class A with greet()	Method greet defined in A	A.greet() available
2	Create class B inheriting A with greet()	Overrides greet()	B.greet() available
3	Create class C inheriting A with greet()	Overrides greet()	C.greet() available
4	Create class D inheriting B and C	No greet() defined	D inherits from B and C
5	Create instance obj of D	obj is instance of D	obj ready
6	Call obj.greet()	Look for greet in D	Not found in D
7	Check B for greet()	Found greet() in B	Use B.greet()
8	Execute B.greet()	Prints 'Hello from B'	Output: Hello from B
9	End	Method call complete	Execution stops

💡 Method greet found in B during MRO, so execution stops after printing.

Variable Tracker

Variable	Start	After Step 5	After Step 6	After Step 7	Final
obj	undefined	instance of D	instance of D	instance of D	instance of D

Key Moments - 3 Insights

Why does obj.greet() call B's greet() and not C's or A's?

Because Python uses Method Resolution Order (MRO) which checks classes in the order D -> B -> C -> A. The execution_table row 7 shows greet() found first in B.

What happens if class D defines its own greet() method?

Then obj.greet() would call D's greet() directly, skipping B and C. This is because the method is found immediately in D (see execution_table row 6).

Why is the diamond problem not causing ambiguity here?

Python's MRO linearizes the inheritance so only one path is chosen. The execution_table shows the order checked, preventing ambiguity.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the greet() method found for obj.greet()?

A Step 7

B Step 6

C Step 8

D Step 4

💡 Hint

Check the 'Evaluation' column in execution_table rows 6 and 7 to see where greet() is found.

According to variable_tracker, what is the value of obj after step 5?

A undefined

B instance of A

C instance of D

D instance of B

💡 Hint

Look at variable_tracker row for obj under 'After Step 5'.

If class D had its own greet() method, how would the execution_table change at step 6?

A greet() found in C at step 7

B greet() found in D at step 6

C greet() found in B at step 7

D No greet() found, error

💡 Hint

Refer to key_moments explanation about method overriding and execution_table step 6.

Concept Snapshot

Diamond problem occurs when a class inherits from two classes that share a common ancestor.
Python solves this using Method Resolution Order (MRO).
MRO defines a linear order to search methods.
In diamond shape, MRO prevents ambiguity by choosing one path.
Use class.__mro__ to see the order.
Method calls follow this order to find the right method.

Full Transcript

This visual trace shows how Python handles the diamond problem in multiple inheritance. We define classes A, B, C, and D where B and C inherit from A, and D inherits from B and C. When we create an instance of D and call greet(), Python looks for the method in D first, then B, then C, then A. It finds greet() in B and executes it, printing 'Hello from B'. This is because Python uses Method Resolution Order (MRO) to avoid ambiguity in diamond inheritance. The variable tracker shows obj is an instance of D throughout. Key moments clarify why B's greet() is chosen and how defining greet() in D would change the behavior. The quizzes test understanding of method lookup steps and variable states. This helps beginners see step-by-step how Python resolves methods in diamond inheritance.