Method Resolution Order (MRO) in Python - Step-by-Step Execution

Concept Flow - Method Resolution Order (MRO)

Define Classes with Inheritance

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Create Instance of Child Class

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Call Method on Instance

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Python Checks MRO List

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Search Method in Classes in MRO Order

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First Found Method is Used

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Method Runs

When you call a method on an object, Python looks for it following a specific order called MRO, checking classes from child to parents.

Execution Sample

Python

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

class B(A):
    pass

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

obj = C()
print(obj.greet())

This code shows how Python finds the greet method in class C before checking parents.

Execution Table

Step	Action	MRO List	Method Found?	Output
1	Create instance obj of class C	[C, B, A, object]	No
2	Call obj.greet()	[C, B, A, object]	Check C	Yes, greet in C
3	Execute greet() from C	[C, B, A, object]	N/A	Hello from C
4	Print output	[C, B, A, object]	N/A	Hello from C

💡 Method greet found in class C, so search stops and method runs.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
obj	undefined	instance of C	instance of C	instance of C	instance of C
MRO	undefined	[C, B, A, object]	[C, B, A, object]	[C, B, A, object]	[C, B, A, object]
method_greet	undefined	undefined	greet from C	greet from C	greet from C
output	undefined	undefined	undefined	Hello from C	Hello from C

Key Moments - 3 Insights

Why does Python use class C's greet method instead of class A's?

Because the execution_table row 2 shows Python checks classes in MRO order starting from C, and finds greet in C first, so it stops searching.

What if class C did not have greet method? Where would Python look next?

Python would continue down the MRO list to class B, then class A, as shown in the concept_flow and execution_table step 2 logic.

Why is object included in the MRO list?

Because all classes in Python inherit from object, the base class, so Python checks it last if method is not found earlier, as shown in the MRO list in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, which class's greet method is found first?

A Class A

B Class B

C Class C

D object

💡 Hint

Check the 'Method Found?' column in execution_table row 2.

According to variable_tracker, what is the MRO list after step 1?

A [C, B, A, object]

B [object, A, B, C]

C [A, B, C, object]

D [B, C, A, object]

💡 Hint

Look at the MRO variable values after step 1 in variable_tracker.

If class C did not have greet method, which class would Python check next according to the MRO?

A object

B B

C A

D None, it would error

💡 Hint

Refer to the MRO list in execution_table and concept_flow to see the order after C.

Concept Snapshot

Method Resolution Order (MRO) in Python:
- Determines the order Python looks for methods in inheritance.
- Starts from the child class, then parents, up to object.
- First class with the method stops the search.
- Use __mro__ attribute to see the order.
- Important for multiple inheritance to avoid confusion.

Full Transcript

This visual trace shows how Python finds methods using Method Resolution Order (MRO). When an object calls a method, Python checks the class of the object first, then its parents in a specific order. In the example, class C has a greet method, so Python uses it without checking parents. The MRO list is [C, B, A, object]. If C did not have greet, Python would check B, then A, then object. This order ensures Python knows exactly which method to run, even with multiple inheritance.