Add missing type hints

[Pennycook]

Related issues

• Closes Add missing type hints #182, @laserkelvin.

Proposed changes

• Configure mypy to check for missing type hints in addition to type errors.
• Add type hints to (almost) all functions, with one exception due to complexity.
• Make minimal code changes to satisfy mypy.

---

I'm sorry that this is so big, but I realized early on that doing this file by file didn't really work (even though the commit history suggests it did). Adding type hints to one file would often cause mypy to go back and re-examine the code that called those functions, potentially introducing a bunch of new errors.

But I think this was worth it, and the code is now much improved. There were quite a few places where adding the type information actually uncovered some bugs (or at least, assumptions) such as assuming that a function returns a value even though it's technically capable of returning None. For the most part, I've satisfied mypy by declaring which functions can sometimes return None, and then adding checks that they don't.

In the long-term, I would like to revisit some of the problems that this change has highlighted, including:

• Rewriting code to avoid | None, by preventing invalid look-ups entirely.
• Rewriting functions to avoid complex nested type definitions (e.g., dicts of lists of tuples of lists...)
• Rewriting file_source (see Rewrite file_source and file_parser #102) to avoid all the complexity related to the file-source Callables.
• Move Platform into the Preprocessor so that we can use strong-typing without introducing a circular import.

[Copilot]

Pull Request Overview

This PR configures mypy to check for missing type hints and adds comprehensive type annotations throughout the codebase to improve type safety and code quality.

• Configures mypy with --disallow-untyped-defs to enforce type annotations
• Adds type hints to almost all functions across the codebase
• Makes minimal code changes to satisfy mypy requirements

Reviewed Changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
.pre-commit-config.yaml	Enables stricter mypy checking and excludes tests from type checking
tests/files/test_filetree_node.py	Updates test assertion to match change from list to set for platforms property
codebasin/tree.py	Adds return type annotations for CLI functions
codebasin/report.py	Comprehensive type hints for reporting functions and classes
codebasin/preprocessor.py	Extensive type annotations for preprocessor tokens, nodes, and parsing classes
codebasin/platform.py	Type hints for platform management functionality
codebasin/language.py	Simple type annotations for file language detection
codebasin/finder.py	Type hints for code analysis and parsing state management
codebasin/file_source.py	Type annotations for file source processing classes
codebasin/file_parser.py	Type hints for file parsing functionality
codebasin/config.py	Type annotations for configuration management
codebasin/_detail/logging.py	Type hints for logging utilities
codebasin/main.py	Type annotations for main entry point functions
codebasin/init.py	Type hints for core data structures

[Pennycook]

I'm choosing to ignore the coverage failures here. A lot of the new lines are actually code that we expect to be unreachable, and designing a test for such cases seems impossible. Additionally, most (and possibly all) of the new lines are part of functionality that needs to be rewritten, and I'd rather focus on providing more robust tests (#96) to guide those efforts.

[swright87]

I don't see any issues here.