Python Typing

Python typing conventions

A static type checker is the type gate for the project: either ty (Astral's fast checker and language server) or pyright (Microsoft's checker, the engine behind Pylance) — pick whichever the project already uses. Type the public surface, keep the type check clean, and write modern annotation syntax. ty is pre-1.0, so expect its diagnostics and config to move. For deeper edge cases see reference.md.

1. Annotate the public surface

Every public function, method, and module-level constant gets parameter and return annotations — that is the contract the checker enforces and editors surface. Annotate internal locals only where the type is not obvious from the right-hand side. Avoid Any (it disables checking — prefer object, a Protocol, or a type variable) and use # type: ignore[rule-code] with a specific code, sparingly, never a bare blanket ignore.

2. Configure and run a type checker

Both checkers configure in pyproject.toml and run through uv. Use one — wire its check into make typecheck and CI.

ty reads [tool.ty]. The minimal config sets the version floor and the src root; tune individual diagnostics under [tool.ty.rules]:

[tool.ty.environment]
python-version = "3.13"
root = ["./src"]

[tool.ty.rules]
possibly-unresolved-reference = "warn"   # severities: "ignore" | "warn" | "error"

uv run ty check            # type-check the project (wire into `make typecheck` + CI)
uv run ty server           # the LSP language server, for editor integration

pyright reads [tool.pyright] (or a standalone pyrightconfig.json, which takes precedence if present). Set the checked paths and the interpreter version; typeCheckingMode selects strictness ("basic" | "standard" | "strict", default "standard"):

[tool.pyright]
include = ["src"]
pythonVersion = "3.13"
typeCheckingMode = "standard"

uv add --dev pyright       # pin it in the dev dependency group
uv run pyright             # type-check the project (or `uvx pyright` for a one-off run)

3. Write modern annotation syntax

• X | None, not Optional[X]; A | B, not Union. Built-in generics (list[int], dict[str, int]) — never the deprecated typing.List/Dict. The ruff UP rules (see python-style) rewrite the old forms.

• PEP 695 type parameters and aliases (3.12+) instead of TypeVar/TypeAlias:

  def first[T](items: list[T]) -> T: ...
  
  class Box[T]:
      def __init__(self, value: T) -> None:
          self.value = value
  
  type Vector = list[float]
  

• typing.Self for fluent/factory methods that return their own type; @override (3.12+) on methods that override a base, so the checker catches signature drift.

4. Prefer Protocol over ABC for interfaces

Structural typing keeps the interface at the consumer (the analogue of Go's consumer-defined interfaces) — no base class to inherit, and any object with the right shape satisfies it:

from typing import Protocol

class Reader(Protocol):
    def read(self, size: int) -> bytes: ...

Reserve abc.ABC for when you need shared implementation or runtime isinstance enforcement.

5. Deferred annotations

Default to from __future__ import annotations at the top of each module: annotations become strings, so forward references need no quotes, import cycles used only for typing disappear, and there is no runtime cost. Guard typing-only imports behind if TYPE_CHECKING:. The exception is code that introspects annotations at runtime (some serializers/validators) — see reference.md.

For runtime data validation reach for a model library; plain annotations are not checked at runtime. For style and the ruff UP rules see the python-style skill; for typing test doubles see python-testing; for the make typecheck target and CI wiring see python-project and python-release.