R23-T021: Make data_structures/linked_list/skip_list.py fully typed - #71

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues, then the description above includes the issue number(s) with a closing keyword: "Fixes Md Sohail — R23-T021: Make data_structures/linked_list/skip_list.py fully typed #3".

Description

Adds accurate parameter and return type annotations to all four unhinted functions in data_structures/linked_list/skip_list.py: __iter__, _locate_node, traverse_keys, and is_sorted, keeping generics consistent with SkipList[KT, VT]. Fixes a real bug exposed in find() where parameter key was typed as VT instead of KT, and adds a doctest verifying lookup on a SkipList[str, int].

Related Issue

Fixes: #3

Type of Change

• Bug Fix
• New Feature
• UI/UX Improvement
• Performance Improvement
• Tests
• Documentation
• Refactoring / Maintenance
• CI/CD

Changes Made

• data_structures/linked_list/skip_list.py:
  • SkipList.__iter__(self) -> Iterator[KT]: Added return type hint using collections.abc.Iterator[KT] and cast(KT, node.forward[0].key) so that __iter__ consistently yields KT.
  • SkipList._locate_node(self, key: KT) -> tuple[Node[KT, VT] | None, list[Node[KT, VT]]]: Annotated parameter key: KT.
  • SkipList.find(self, key: KT) -> VT | None: Fixed parameter type from VT to KT (exposed by type checking _locate_node(key)), and added a doctest verifying lookup on SkipList[str, int].
  • traverse_keys(node: Node[str, int]) -> Iterator[str]: Inside test_delete_doesnt_leave_dead_nodes(), annotated parameter and return with concrete types.
  • is_sorted(lst: list[int]) -> bool: Inside test_iter_always_yields_sorted_values(), annotated parameter as list[int] and return type as bool.
• work/R23-T021/: Added FINDINGS.md, ty_before.txt, ty_after.txt, and test_output.txt documenting before/after type checking runs and test results.

Screenshots / Recordings

Not applicable (no UI changes).

Testing

• uv run --python 3.15 --with ty ty check data_structures/linked_list/skip_list.py -> 0 diagnostics for the file.
• uv run --python 3.15 pytest -v data_structures/linked_list/skip_list.py -> 16/16 passed.
• uv run --python 3.15 python -m doctest -v data_structures/linked_list/skip_list.py -> 36/36 passed.

Additional Notes

• Findings:
  1. In SkipList.__iter__, node.forward[0].key carries type KT | str because Node.key defaults to "root" on self.head. Using cast(KT, ...) reflects that only user-inserted nodes (key: KT) exist in node.forward.
  2. Annotating _locate_node(key: KT) directly exposed a type bug in find(key: VT): find was passing key: VT into _locate_node(key: KT). Correcting find(key: KT) resolves this issue.
  3. A second branch R23-T021-upstream containing only the code change (no work/ folder) has been created and pushed for a later upstream PR.