Quick Answer
The answer is that an AttributeError is raised. This happens because when a class defines the __slots__ attribute, it restricts attribute assignment to only the names explicitly listed in that tuple or list, such as 'x' and 'y' in this case. The __slots__ attribute tells Python not to create a per-instance __dict__, which is the usual dictionary that stores instance attributes, thereby saving memory. Attempting to assign an attribute like 'z' that is not in __slots__ triggers an AttributeError, as the instance has no place to store it. On the Certified Associate Python Programmer PCAP exam, this tests your understanding of memory optimization and attribute restriction in classes; a common trap is assuming you can still add dynamic attributes to slotted objects. Remember the mnemonic: slots lock the attribute list—if it’s not slotted in, it’s locked out.
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 class 'Point' is defined with __slots__ = ['x', 'y']. A developer creates an instance p = Point() and tries to set p.z = 10. What happens?
The assignment is silently ignored
Why wrong: It raises an error.
Python issues a warning and adds the attribute
Why wrong: No warning; raises AttributeError.
The attribute 'z' is added dynamically
Why wrong: Slots prevents dynamic attributes.
AttributeError is raised
Slots disallow adding new attributes.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
AttributeError is raised
When a class defines `__slots__`, it restricts attribute assignment to only those names listed in the tuple. Attempting to assign an attribute not in `__slots__` raises an `AttributeError`. This is a deliberate memory optimization that prevents the creation of a per-instance `__dict__`, so dynamic attributes are disallowed.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The assignment is silently ignored
Why it's wrong here
It raises an error.
Python issues a warning and adds the attribute
Why it's wrong here
No warning; raises AttributeError.
The attribute 'z' is added dynamically
Why it's wrong here
Slots prevents dynamic attributes.
AttributeError is raised
Why this is correct
Slots disallow adding new attributes.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Python Institute often tests the misconception that `__slots__` only provides a hint or that attributes can still be added dynamically, when in fact it strictly forbids any attribute not listed in `__slots__`.
Detailed technical explanation
Under the hood, `__slots__` reserves space in the instance for the listed attributes using descriptors, eliminating the per-instance `__dict__` and reducing memory overhead. A subtle behavior is that if `__slots__` is defined in a subclass but not in the parent, the subclass instance may still have a `__dict__` unless explicitly excluded. In real-world scenarios, this is critical for high-performance applications like game engines or data processing where millions of objects must minimize memory footprint.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Object-Oriented Programming — This question tests Object-Oriented Programming — Read the scenario before looking for a memorised answer..
The correct answer is: AttributeError is raised — When a class defines `__slots__`, it restricts attribute assignment to only those names listed in the tuple. Attempting to assign an attribute not in `__slots__` raises an `AttributeError`. This is a deliberate memory optimization that prevents the creation of a per-instance `__dict__`, so dynamic attributes are disallowed.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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