Quick Answer
The correct answer is D, B, C, A, object. This order is determined by Python’s C3 linearization algorithm, which resolves the diamond inheritance pattern by merging the linearizations of D’s parents—B and C—while preserving local precedence order and monotonicity. For class D(B, C), the merge of L(B) = [B, A, object] and L(C) = [C, A, object] with the parent list [B, C] yields D first, then B (since it appears first in the parent list), then C, then A (the common ancestor), and finally object. On the PCAP exam, this tests your understanding of how Python avoids ambiguity in multiple inheritance, often appearing as a code-tracing question where the trap is assuming depth-first, left-to-right order (which would give D, B, A, C, object). A handy memory tip: think of C3 as “children before parents, and left parents before right parents”—so D, then B before C, then their shared parent A, then object.
This PCAP practice question tests your understanding of object-oriented programming. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Consider the following code snippet: 'class A: pass; class B(A): pass; class C(A): pass; class D(B, C): pass'. What is the Method Resolution Order (MRO) for class D according to the C3 linearization algorithm used by Python?
D, B, A, C, object
Why wrong: This would be depth-first but does not satisfy monotonicity.
D, B, C, A
Why wrong: MRO always ends with object.
D, B, A, object, C
Why wrong: C is a parent and must appear before object.
D, B, C, A, object
Correct linearization based on C3.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
D, B, C, A, object
The C3 linearization algorithm merges the linearizations of D's parents (B and C) with their parent list, respecting the local precedence order and monotonicity. For class D(B, C), the MRO is computed as D + merge(L(B), L(C), [B, C]), where L(B) = B, A, object and L(C) = C, A, object. The merge yields D, B, C, A, object, which is the correct MRO.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
D, B, A, C, object
Why it's wrong here
This would be depth-first but does not satisfy monotonicity.
D, B, C, A
Why it's wrong here
MRO always ends with object.
D, B, A, object, C
Why it's wrong here
C is a parent and must appear before object.
D, B, C, A, object
Why this is correct
Correct linearization based on C3.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Python Institute often tests the misconception that Python uses depth-first left-to-right resolution (like in old-style classes), leading candidates to pick Option A (D, B, A, C, object) instead of the correct C3 linearization result.
Detailed technical explanation
The C3 linearization algorithm ensures consistent MRO by merging parent linearizations using a head-tail comparison, rejecting any head that appears in the tail of another list. This prevents the 'diamond problem' and guarantees that subclasses override methods in a predictable order. In practice, understanding MRO is critical when using cooperative multiple inheritance with super(), as the order determines which method is called next.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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FAQ
Object-Oriented Programming — This question tests Object-Oriented Programming — Read the scenario before looking for a memorised answer..
The correct answer is: D, B, C, A, object — The C3 linearization algorithm merges the linearizations of D's parents (B and C) with their parent list, respecting the local precedence order and monotonicity. For class D(B, C), the MRO is computed as D + merge(L(B), L(C), [B, C]), where L(B) = B, A, object and L(C) = C, A, object. The merge yields D, B, C, A, object, which is the correct MRO.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Consider the following code: class A: x = 1; class B(A): pass; class C(A): x = 2; class D(B, C): pass. What is D.x?
mediumWhy C: Option C is correct because Python's method resolution order (MRO) for class D, which uses multiple inheritance, follows the C3 linearization algorithm. The MRO for D is D, B, C, A, and since C defines x = 2, that attribute overrides the x = 1 from A in the inheritance chain, so D.x resolves to 2.
Variation 2. Consider the following class hierarchy: class A: def method(self): return 'A'; class B(A): pass; class C(A): def method(self): return 'C'; class D(B, C): pass. What is the output of D().method() according to Python's MRO?
mediumWhy D: Python's MRO (Method Resolution Order) for class D(B, C) is computed using the C3 linearization algorithm, which respects the local precedence order and monotonicity. The MRO for D is D -> B -> C -> A, so D().method() resolves to C.method(), returning 'C'. Option D is correct because C is the first class in the MRO that defines method().
Variation 3. A developer defines a class hierarchy with multiple inheritance: class A, class B(A), class C(A), class D(B,C). The method 'm' is defined only in A. What is the method resolution order for D according to the C3 linearization algorithm?
mediumWhy C: C is correct because the C3 linearization algorithm merges the linearizations of parent classes while preserving local precedence order and monotonicity. For class D(B, C), the linearization is D + merge(L(B), L(C), [B, C]). L(B) = B, A, object; L(C) = C, A, object. Merging yields D, B, C, A, object (object is often omitted in option lists). This respects that B precedes C in D's bases and that A appears after both B and C.
Variation 4. Given: class A: def method(self): print('A'); class B(A): def method(self): super().method(); print('B'); class C(A): def method(self): super().method(); print('C'); class D(B, C): pass. What is printed by D().method()?
mediumWhy B: Option B is correct because Python's MRO (Method Resolution Order) for class D, which inherits from B and C (both inheriting from A), follows the C3 linearization algorithm. The MRO for D is D -> B -> C -> A, so calling D().method() triggers B.method(), which calls super().method() (resolving to C.method()), which calls super().method() (resolving to A.method()), printing 'A', then back to C prints 'C', then back to B prints 'B', resulting in 'A C B'.
Variation 5. Refer to the exhibit. Which of the following correctly shows the MRO of class D?
mediumWhy A: Option A is correct because Python's Method Resolution Order (MRO) for class D, which inherits from B and C (where B inherits from A and C inherits from A), follows the C3 linearization algorithm. The MRO is computed as D -> B -> C -> A -> object, ensuring that each class appears before its parents and that the order respects the local precedence order of D's bases (B before C).
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Last reviewed: Jun 30, 2026
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