The answer is to use a custom descriptor that caches the value in the instance's __dict__. This pattern, known as the cached property descriptor pattern, is correct because the descriptor’s __get__ method computes the attribute on first access and then stores the result directly in the instance’s __dict__, so subsequent lookups bypass the descriptor entirely and retrieve the cached value. On the Certified Associate Python Programmer PCAP exam, this tests your understanding of how descriptors interact with instance dictionaries and the attribute lookup chain—a common trap is confusing a cached property with a simple @property, which recomputes on every call. Remember the key insight: the descriptor writes into the instance’s __dict__, effectively “shadowing” itself. A useful memory tip is “compute once, store in __dict__” to distinguish it from a regular property.
PCAP Object-Oriented Programming Practice Question
This PCAP practice question tests your understanding of object-oriented programming. Compare every option against the stated constraints before choosing — the best answer satisfies all requirements, not just the most obvious one. 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.
A class has an attribute that should be computed on access and cached for subsequent accesses. Which pattern is most appropriate?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a custom descriptor that caches the value in the instance's __dict__.
Option B is correct because a custom descriptor with a __get__ method can compute the attribute value on first access and store it in the instance's __dict__ for subsequent accesses. This pattern, often called a cached property, ensures the computation happens only once per instance, which is exactly what the question requires.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use __slots__ to define the attribute.
Why it's wrong here
__slots__ restricts attributes but does not provide caching behavior.
✓
Use a custom descriptor that caches the value in the instance's __dict__.
Why this is correct
A descriptor can compute on first access and store the result for subsequent calls.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use a @classmethod to compute the value.
Why it's wrong here
Classmethods do not have access to instance state for caching.
✗
Use a @staticmethod to compute the value.
Why it's wrong here
Static methods cannot access instance or class state.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Python Institute often tests the distinction between descriptors that compute on every access versus those that cache, and the trap here is that candidates confuse __slots__ (memory optimization) with caching, or think that class-level methods like @classmethod can somehow provide per-instance caching.
Detailed technical explanation
How to think about this question
Under the hood, a descriptor that caches in __dict__ works by overriding __get__: on first access, it computes the value, then stores it via instance.__dict__[attr_name] = value, and returns it. Subsequent accesses find the value directly in __dict__ without invoking the descriptor again. This pattern is used in real-world frameworks like Django's cached_property decorator and Python's functools.cached_property (Python 3.8+), which also handle thread safety and attribute deletion correctly.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Object-Oriented Programming — This question tests Object-Oriented Programming — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use a custom descriptor that caches the value in the instance's __dict__. — Option B is correct because a custom descriptor with a __get__ method can compute the attribute value on first access and store it in the instance's __dict__ for subsequent accesses. This pattern, often called a cached property, ensures the computation happens only once per instance, which is exactly what the question requires.
What should I do if I get this PCAP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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