The correct answer is that using __slots__ prevents the automatic creation of an instance __dict__. This happens because when you define __slots__ in a class, Python generates fixed descriptors for each listed attribute, which blocks the dynamic allocation of a per-instance dictionary. As a result, any attempt to assign an attribute not named in __slots__ raises an AttributeError, enforcing a strict, memory-efficient attribute set. On the Certified Associate Python Programmer PCAP exam, this concept tests your understanding of attribute management and memory optimization—a common trap is assuming __slots__ still allows a __dict__ unless explicitly included. Remember the mnemonic: “Slots lock the slots—no dict, no extra tricks.”
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.
Which TWO statements about the use of __slots__ in a class are correct?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It restricts instance attributes to only those named in __slots__
Option C is correct because when you define `__slots__` in a class, Python creates a descriptor for each slot name and restricts the instance to only those attributes. Any attempt to set an attribute not listed in `__slots__` raises an `AttributeError`. This is a core feature of `__slots__` to enforce a fixed set of instance attributes.
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.
✗
It prevents the class from being instantiated
Why it's wrong here
__slots__ does not prevent instantiation; it just changes attribute storage.
✗
It is inherited by subclasses automatically
Why it's wrong here
Subclasses must define their own __slots__ to avoid __dict__; they don't inherit __slots__ fully.
✓
It restricts instance attributes to only those named in __slots__
Why this is correct
Assigning an attribute not in __slots__ raises AttributeError.
Related concept
Read the scenario before looking for a memorised answer.
✗
It improves method lookup speed
Why it's wrong here
Method lookup is unaffected; __slots__ only affects attribute access.
✓
It prevents automatic creation of an instance __dict__
Why this is correct
__slots__ declares a fixed set of attributes, omitting __dict__ to save memory.
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 `__slots__` is inherited by subclasses automatically, but in reality, subclasses must explicitly define their own `__slots__` to avoid inheriting a `__dict__`.
Detailed technical explanation
How to think about this question
Under the hood, `__slots__` creates descriptors (like `member_descriptor`) for each slot name, which store values in a compact array-like structure on the instance, bypassing the instance `__dict__`. This reduces memory usage significantly when creating many instances, as each instance saves the overhead of a `__dict__` (about 64 bytes per instance in CPython). A subtle behavior: if a class defines `__slots__` and a subclass does not, the subclass instances will have a `__dict__` in addition to the inherited slots, negating the memory benefit.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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: It restricts instance attributes to only those named in __slots__ — Option C is correct because when you define `__slots__` in a class, Python creates a descriptor for each slot name and restricts the instance to only those attributes. Any attempt to set an attribute not listed in `__slots__` raises an `AttributeError`. This is a core feature of `__slots__` to enforce a fixed set of instance attributes.
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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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.
Question Discussion
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