Type Checking

The compile-time gate that lint can't do

Type checking is what catches bugs lint cannot — wrong-type arguments, missing fields, unhandled None / null, mismatches between API contracts. It runs at the boundary between 'static analysis' and 'simulated execution' — it walks every code path mentally so you don't have to.

The major checkers

Python — mypy: gradual typing, the original. Reads PEP 484 type hints. Strict mode is recommended (--strict).
Python — pyright: Microsoft's. Faster, more accurate inference, powers Pylance in VS Code. Often used alongside ruff.
TypeScript — tsc: tsc --noEmit just type-checks without producing output. The canonical gate.
JavaScript with JSDoc — yes, JSDoc + // @ts-check buys you most of TypeScript's checking without a transpile step.

Type checking in CI

Make it a separate job (parallel with test, lint).
Pin the checker version — type errors change between releases.
Output should be both human-readable and machine-readable when you want annotations on the PR diff.
For Python, run on the same Python version as production.

Code

Parallel type-check jobs (Python + TypeScript)·yaml

  type-check-python:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: astral-sh/setup-uv@v3
      - run: uv sync --all-extras --dev
      - run: uv run mypy src --strict --no-incremental

  type-check-typescript:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: pnpm/action-setup@v4
        with: { version: 9 }
      - uses: actions/setup-node@v4
        with: { node-version: '22', cache: pnpm }
      - run: pnpm install --frozen-lockfile
      - run: pnpm tsc --noEmit

The compile-time gate that lint can't do

The major checkers

Type checking in CI

Code

External links

Exercise

Progress

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