- A
Add a lock around the cache lookup and creation in the factory function
Why wrong: This prevents double creation of objects but does not prevent double processing if the same object is processed twice.
- B
Add a flag to each transaction object to indicate if it has been processed, and check it at the start of process()
Why wrong: This could work but is not thread-safe without proper synchronization; also adds complexity.
- C
Remove the caching mechanism from the factory function to ensure new objects are always created
Why wrong: This may cause other issues like memory leaks and does not address the root cause of double processing.
- D
Make the process() method idempotent by checking if the transaction has already been applied to the account (e.g., check balance changes)
Idempotency ensures that repeated calls do not cause duplicate effects, which is the safest approach in a multi-threaded environment.
Quick Answer
The answer is to make the process() method idempotent by checking if the transaction has already been applied to the account, such as verifying balance changes. This is correct because idempotent method design to avoid duplicate processing ensures that even if the same transaction object is passed multiple times—due to the factory’s caching and concurrent thread access—the financial effect occurs only once. On the PCAP exam, this tests your understanding of concurrency pitfalls and the principle of idempotency in object-oriented design, often disguised in a scenario about caching or threading. A common trap is to focus on fixing the factory’s cache (e.g., adding locks), but that only prevents duplicate objects, not duplicate processing of the same object. Remember: idempotency is about making repeated calls safe, not preventing the repeats themselves. Memory tip: “Check before you double—idempotent methods keep accounts single.”
PCAP Object-Oriented Programming Practice Question
This PCAP practice question tests your understanding of object-oriented programming. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
You are working on a legacy system that processes financial transactions. The system uses a class hierarchy: Transaction (base), Deposit, Withdrawal, Transfer. Each subclass overrides a method 'process()' to handle its specific logic. The code often runs in a multi-threaded environment and you notice intermittent errors where a transaction is processed twice. The logging shows that the same transaction object is being passed to the process method multiple times. The transaction objects are created from a factory function that caches recently used transactions. The errors seem to occur when two threads call the factory at the same time with the same parameters. After investigating, you find that the factory uses a class-level dictionary to cache objects. Which of the following is the most appropriate solution to prevent double processing?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Make the process() method idempotent by checking if the transaction has already been applied to the account (e.g., check balance changes)
Option D is correct because the core issue is that the same transaction object can be processed multiple times in a multi-threaded environment, even if the factory is fixed. Making process() idempotent by checking whether the transaction has already been applied (e.g., verifying account balance changes) ensures that repeated calls with the same object do not cause duplicate financial effects, directly addressing the symptom of double processing regardless of how the object is cached or retrieved.
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.
- ✗
Add a lock around the cache lookup and creation in the factory function
Why it's wrong here
This prevents double creation of objects but does not prevent double processing if the same object is processed twice.
- ✗
Add a flag to each transaction object to indicate if it has been processed, and check it at the start of process()
Why it's wrong here
This could work but is not thread-safe without proper synchronization; also adds complexity.
- ✗
Remove the caching mechanism from the factory function to ensure new objects are always created
Why it's wrong here
This may cause other issues like memory leaks and does not address the root cause of double processing.
- ✓
Make the process() method idempotent by checking if the transaction has already been applied to the account (e.g., check balance changes)
Why this is correct
Idempotency ensures that repeated calls do not cause duplicate effects, which is the safest approach in a multi-threaded environment.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Python Institute often tests the misconception that preventing object reuse or adding locks in the factory is sufficient to fix double processing, when the real requirement is to make the operation itself idempotent to handle any scenario where the same object is processed more than once.
Detailed technical explanation
How to think about this question
Idempotency is a critical design pattern in financial systems, often implemented by recording a unique transaction ID in a database with a unique constraint, so that duplicate process() calls result in no-ops or rollbacks. Under the hood, this leverages ACID properties (specifically atomicity and consistency) to ensure that even if the same transaction object is processed twice, the second call detects that the effect (e.g., a balance change) has already been applied, often by checking a transaction log or a version field on the account. In real-world systems like banking APIs, idempotency keys are mandatory to prevent duplicate debits or credits, especially in distributed or multi-threaded environments.
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 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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PCAP question test?
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: Make the process() method idempotent by checking if the transaction has already been applied to the account (e.g., check balance changes) — Option D is correct because the core issue is that the same transaction object can be processed multiple times in a multi-threaded environment, even if the factory is fixed. Making process() idempotent by checking whether the transaction has already been applied (e.g., verifying account balance changes) ensures that repeated calls with the same object do not cause duplicate financial effects, directly addressing the symptom of double processing regardless of how the object is cached or retrieved.
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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Last reviewed: Jun 30, 2026
This PCAP practice question is part of Courseiva's free Python Institute certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCAP exam.
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