PCAP Object-Oriented Programming Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  __getitem__ and __contains__

  Why it's wrong here

  Adding `__contains__` enables the `in` operator but does not provide positional access; the sequence protocol minimally requires `__len__` for `len()` and `__getitem__` for indexing. A container can be fully sequence-like for reading without any membership override, because `__contains__` is optional and falls back to a linear scan when absent. Replacing `__len__` with `__contains__` leaves the object unable to report its size or support index-based access, so it fails the core sequence contract.

• ✗

  __setitem__ and __delitem__

  Why it's wrong here

  `__setitem__` and `__delitem__` only handle mutation operations (`obj[i] = value` and `del obj[i]`), so they belong to the mutable-sequence interface, not the read-only sequence protocol. Without `__getitem__` and `__len__`, the object has no way to fetch a value by position or report a length, making even basic indexing impossible. Adding mutation methods cannot substitute for the two read-access dunders required for sequence behavior.

• ✗

  __iter__ and __next__

  Why it's wrong here

  `__iter__` and `__next__` create an iterator protocol that supports `for` loops and `next()`, but they do not make integer subscription work; `obj[i]` is handled by `__getitem__`, not by iteration. In fact, if `__getitem__` is present, Python can iterate by calling it with increasing indices until `IndexError`, so `__iter__` is not even required for iteration. The absence of `__len__` and `__getitem__` means the object cannot be measured with `len()` nor positionally indexed, so it is not a sequence.

• ✓

  __getitem__ and __len__

  Why this is correct

  According to the Python data model, a sequence is defined primarily by having a `__len__` method and a `__getitem__` method that accepts integer indices (and optionally slices). Together they give the object a finite length, support `obj[i]` indexing, and implicitly enable iteration via the old-style `__getitem__` fallback. On top of these two, the `collections.abc.Sequence` ABC automatically mixes in `__contains__`, `__iter__`, `__reversed__`, `index`, and `count`, showing how much behavior is derived from just these two methods.