Implementing Sequence Protocol for Slicing
You are designing a class that should behave like a sequence and support slicing. Which special methods must be implemented?
Quick Answer
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.
⚠ Common exam trap
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.
Answer choices
Why each option matters
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]`.
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.
- ✗
__getitem__ and __setitem__
Why it's wrong here
__setitem__ is for item assignment, not necessary for read-only sequences.
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Same concept, more angles
1 more way this is tested on PCAP
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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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