python-type-safety

Annotate all public function parameters, return types, and class attributes first, then run mypy or pyright to find gaps. For existing projects, enable strict mode incrementally using per-module overrides in pyproject.toml rather than converting everything at once.

Use `T | None` (modern union syntax) for Python 3.10 and later, as it is cleaner and preferred. Use `Optional[T]` from the typing module only when supporting Python 3.9 or earlier.

Protocol provides structural typing where a class satisfies the interface by matching method signatures without inheriting anything. ABC requires explicit inheritance. Use Protocol for duck-typed interfaces and ABC when you need shared implementation.

No, the `type Alias = ...` statement (PEP 695) requires Python 3.12. For Python 3.10 and 3.11, use `TypeAlias` from the typing module, e.g., `UserId: TypeAlias = str`.

Type narrowing only works when the guard is visible to the type checker in the same scope. After `if user is None: raise ...`, mypy narrows the type to the non-None branch, but narrowing is lost across function calls or when mutating captured variables.

Use `Any` only for truly dynamic data or when interfacing with untyped third-party libraries. Prefer specific types, generics, or TypeVar bounds elsewhere, since `Any` disables type checking for that value entirely.