The answer is `__getitem__` and `__len__`. These two special methods form the core of the sequence protocol in Python, which is what enables slicing behavior like `obj[start:stop:step]`. The `__getitem__` method must handle both integer indices and slice objects, while `__len__` provides the sequence’s length, which Python uses internally to compute proper slice boundaries and defaults. On the Certified Associate Python Programmer PCAP exam, this topic tests your understanding of how Python’s data model supports custom sequences—a common trap is thinking that only `__getitem__` is needed, but without `__len__`, slicing can produce incorrect or unexpected results, especially with negative indices or omitted bounds. A reliable memory tip is to remember that “slicing needs both a map and a measure”: `__getitem__` maps indices to values, and `__len__` measures the total size, so Python knows where to start and stop.
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.
You are designing a class that should behave like a sequence and support slicing. Which special methods must be implemented?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
__getitem__ and __len__
For a class to support slicing in Python, it must implement both `__getitem__` (to handle indexing and slice objects) and `__len__` (to define the sequence length, which is required for proper slice boundary handling). Together, these satisfy the sequence protocol, enabling Python's slicing syntax like `obj[start:stop:step]`.
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.
✗
__len__ and __contains__
Why it's wrong here
__contains__ is for membership, not slicing.
✗
__iter__ and __next__
Why it's wrong here
These support iteration, not slicing directly.
✗
__getitem__ alone
Why it's wrong here
While __getitem__ is essential, sequences are expected to have __len__ as well.
✓
__getitem__ and __len__
Why this is correct
These two methods are the minimum for a sequence that supports slicing.
Related concept
Read the scenario before looking for a memorised answer.
✗
__getitem__ and __setitem__
Why it's wrong here
__setitem__ is for item assignment, not necessary for read-only sequences.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Python Institute often tests the misconception that `__getitem__` alone is enough for slicing, but the trap is that `__len__` is also required for the interpreter to handle slice defaults and negative indices correctly.
Detailed technical explanation
How to think about this question
Under the hood, when Python encounters a slice like `obj[1:5]`, it creates a `slice` object and passes it to `__getitem__`. The `__len__` method is used by the interpreter to resolve negative indices and to compute default slice boundaries (e.g., `start=0` if omitted). A real-world scenario is implementing a custom immutable sequence (like a read-only log) where you must support slicing without allowing modification, so `__setitem__` is intentionally omitted.
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 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.
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: __getitem__ and __len__ — For a class to support slicing in Python, it must implement both `__getitem__` (to handle indexing and slice objects) and `__len__` (to define the sequence length, which is required for proper slice boundary handling). Together, these satisfy the sequence protocol, enabling Python's slicing syntax like `obj[start:stop:step]`.
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.
About these practice questions
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Variation 1. A Python developer is implementing a class that should behave like a sequence and support indexing. Which pair of special methods must be defined to achieve this?
hard
A.__getitem__ and __contains__
B.__setitem__ and __delitem__
C.__iter__ and __next__
✓ D.__getitem__ and __len__
Why D: To make a class behave like a sequence and support indexing (e.g., obj[0]), Python requires the __getitem__ method to retrieve items by key. Additionally, the __len__ method is needed to define the length of the sequence, which is used by built-in functions like len() and is part of the sequence protocol. Together, these two methods satisfy the minimal requirements for a sequence-like object that supports indexing.
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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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