Quick Answer
The answer is __getitem__, __len__, and __iter__, though the critical method for both iteration and membership in an immutable object is __getitem__. This is correct because implementing __getitem__ allows Python to fall back to sequential indexing for iteration when __iter__ is absent, and it also enables the 'in' operator for membership testing when __contains__ is not defined. On the PCAP exam, this tests your understanding of Python's iteration protocol and the fallback behavior between __iter__ and __getitem__—a common trap is assuming __iter__ is always required for iteration, when in fact __getitem__ alone suffices for immutable sequences. For membership, Python will iterate through the object using __getitem__ to check each element, making it a dual-purpose method. A useful memory tip: "Get item, get iteration, get membership"—if you implement __getitem__ on an immutable object, you cover both features without needing __contains__ or __iter__, though adding __len__ improves performance for membership checks.
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 THREE methods must be implemented to create an immutable object that supports iteration and membership testing?
__init__
Why wrong: __init__ initializes the object but is not required for iteration or membership.
__getitem__
Supports indexing and slicing, and can be used for iteration.
__len__
Provides the length, needed for iteration boundaries.
__contains__
Why wrong: Optional; membership can be implemented via __iter__.
__iter__
Returns an iterator, needed for for loops and membership (if __contains__ not defined).
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
__getitem__
Option B is correct because implementing __getitem__ allows indexing and slicing, which is essential for iteration in Python when __iter__ is not defined; Python falls back to __getitem__ for iteration. This method also supports membership testing via the 'in' operator when __contains__ is absent, as Python will iterate through the object to check membership.
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.
__init__
Why it's wrong here
__init__ initializes the object but is not required for iteration or membership.
__getitem__
Why this is correct
Supports indexing and slicing, and can be used for iteration.
Related concept
Read the scenario before looking for a memorised answer.
__len__
Why this is correct
Provides the length, needed for iteration boundaries.
Related concept
Read the scenario before looking for a memorised answer.
__contains__
Why it's wrong here
Optional; membership can be implemented via __iter__.
__iter__
Why this is correct
Returns an iterator, needed for for loops and membership (if __contains__ not defined).
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Python Institute often tests the fallback behavior of Python's iteration and membership protocols, leading candidates to incorrectly assume __contains__ is mandatory for membership testing when it is actually optional if __iter__ or __getitem__ is implemented.
Detailed technical explanation
In Python, an immutable object must prevent state changes after creation, typically by overriding __setattr__ and __delattr__ to raise TypeError, or by using a tuple-like structure. For iteration, if __iter__ is not defined, Python's iteration protocol checks for __getitem__ with integer indices starting at 0 until IndexError is raised. Membership testing via 'in' first tries __contains__, then falls back to iteration; thus, implementing __getitem__ and __len__ (for efficient iteration termination) satisfies both iteration and membership without __contains__.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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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: __getitem__ — Option B is correct because implementing __getitem__ allows indexing and slicing, which is essential for iteration in Python when __iter__ is not defined; Python falls back to __getitem__ for iteration. This method also supports membership testing via the 'in' operator when __contains__ is absent, as Python will iterate through the object to check membership.
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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