The correct answer is a tuple of strings like `('x', 'y')` assigned to `__slots__`, because `__slots__` must be set to an iterable of attribute names as strings, and a tuple is a perfectly valid immutable iterable that restricts instance attribute creation to exactly `x` and `y`. Any attempt to assign an attribute outside this tuple will raise an `AttributeError`, which is the core mechanism of attribute restriction. On the Certified Associate Python Programmer PCAP exam, this concept tests your understanding of memory optimization and attribute control in class definitions; a common trap is using a list or a single string instead of a proper iterable of strings. Remember that `__slots__` does not inherit automatically unless explicitly redefined in a subclass, and the iterable must contain only string names. A helpful memory tip: think of `__slots__` as a "locked guest list" — only the names you put inside the parentheses are allowed in.
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 of the following correctly uses `__slots__` to restrict attribute creation to only `x` and `y`?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
`class Foo: __slots__ = ('x', 'y')`
Option D is correct because `__slots__` must be assigned an iterable of strings, and a tuple of strings like `('x', 'y')` is a valid iterable that restricts attribute creation to exactly `x` and `y`. Any attempt to assign an attribute not in this tuple will raise an `AttributeError`.
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.
✗
`class Foo: __slots__ = 'x'`
Why it's wrong here
Assigning a string creates a single slot named 'x' (since strings are iterable), not two slots.
✗
`class Foo: __slots__ = ('x')`
Why it's wrong here
A single-element tuple must have a trailing comma; otherwise it's just a string in parentheses.
✗
`class Foo: __slots__ = ['x', 'y']`
Why it's wrong here
While this works technically, the standard convention is to use a tuple, and the official documentation uses tuples.
✓
`class Foo: __slots__ = ('x', 'y')`
Why this is correct
This correctly defines slots as a tuple, restricting instances to only `x` and `y` attributes.
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 a single string or a parenthesized string without a trailing comma is a valid iterable for `__slots__`, leading candidates to pick options that inadvertently restrict attributes to individual characters rather than the intended attribute names.
Detailed technical explanation
How to think about this question
Under the hood, `__slots__` creates descriptors for each named attribute and prevents the creation of a `__dict__` instance dictionary, saving memory and speeding attribute access. A subtle behavior: if a class inherits from a non-slotted class, the `__dict__` is still created unless the parent also defines `__slots__`. In real-world scenarios, `__slots__` is used in data-heavy applications (e.g., machine learning datasets) to reduce memory footprint per instance.
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: `class Foo: __slots__ = ('x', 'y')` — Option D is correct because `__slots__` must be assigned an iterable of strings, and a tuple of strings like `('x', 'y')` is a valid iterable that restricts attribute creation to exactly `x` and `y`. Any attempt to assign an attribute not in this tuple will raise an `AttributeError`.
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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