- A
Use a lock (threading.Lock) in __new__ to serialize access
A lock ensures that only one thread executes the creation block at a time, making it thread-safe.
- B
Use a class method get_instance() that checks a class variable and creates the instance if needed, and call that from __init__
Why wrong: This does not prevent multiple __new__ calls; the class method approach is not inherently thread-safe without locking.
- C
Use a metaclass that overrides __call__ to return the singleton
Why wrong: Even with a metaclass, without synchronization, multiple threads can still create separate instances.
- D
Override __init__ to check if the instance was already initialized and if so, skip initialization
Why wrong: This does not prevent multiple instances; __new__ returns a new object each time without lock.
Quick Answer
The correct answer is to use a threading.Lock inside __new__ to serialize access. This works because the race condition in a thread-safe singleton arises when multiple threads simultaneously check cls._instance, find it None, and both proceed to create a new instance before either can store it. By wrapping the instance creation logic within a lock acquired at the start of __new__, only one thread can execute the critical section at a time, ensuring that the singleton is truly created once and then returned for all subsequent calls. On the PCAP exam, this tests your understanding of both the singleton design pattern and Python’s threading module, often appearing as a scenario where a junior developer overlooks thread safety. A common trap is thinking that simply checking cls._instance twice (double-checked locking) is enough without a lock, but Python’s GIL does not protect against this specific race condition. Memory tip: “Lock before you check, or two threads will wreck.”
PCAP Object-Oriented Programming Practice Question
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.
A junior developer writes a class 'Logger' that should only ever have one instance (singleton). They attempt to implement it by overriding __new__ to always return the same instance. However, when multiple threads attempt to create a Logger, they sometimes get different instances. Which modification will make the singleton thread-safe?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"always"Why it matters: Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.
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
Use a lock (threading.Lock) in __new__ to serialize access
Option A is correct because the race condition occurs when multiple threads simultaneously check `cls._instance` and find it `None`, then both proceed to create a new instance. Wrapping the creation logic inside a `threading.Lock` in `__new__` ensures that only one thread can execute the critical section at a time, guaranteeing that only one instance is ever created.
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 a lock (threading.Lock) in __new__ to serialize access
Why this is correct
A lock ensures that only one thread executes the creation block at a time, making it thread-safe.
Clue confirmation
The clue word "always" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Use a class method get_instance() that checks a class variable and creates the instance if needed, and call that from __init__
Why it's wrong here
This does not prevent multiple __new__ calls; the class method approach is not inherently thread-safe without locking.
- ✗
Use a metaclass that overrides __call__ to return the singleton
Why it's wrong here
Even with a metaclass, without synchronization, multiple threads can still create separate instances.
- ✗
Override __init__ to check if the instance was already initialized and if so, skip initialization
Why it's wrong here
This does not prevent multiple instances; __new__ returns a new object each time without lock.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Python Institute often tests the misconception that simply overriding `__new__` or using a class method is sufficient for thread safety, when in fact the race condition in the check-then-create pattern requires explicit synchronization like a lock.
Detailed technical explanation
How to think about this question
Under the hood, Python's `__new__` is called before `__init__` and is responsible for returning the instance. Without a lock, the check `if cls._instance is None` is a classic time-of-check-to-time-of-use (TOCTOU) race condition. Using `threading.Lock` with a `with` statement serializes the creation, and the double-checked locking pattern (check without lock, then with lock) is a common optimization to avoid acquiring the lock on every call after the singleton is initialized.
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.
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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: Use a lock (threading.Lock) in __new__ to serialize access — Option A is correct because the race condition occurs when multiple threads simultaneously check `cls._instance` and find it `None`, then both proceed to create a new instance. Wrapping the creation logic inside a `threading.Lock` in `__new__` ensures that only one thread can execute the critical section at a time, guaranteeing that only one instance is ever created.
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.
Are there clue words in this question I should notice?
Yes — watch for: "always". Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.
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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