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Exercise 2 — Build a CI Pipeline

TL;DR — In ~20 minutes you'll build ci.yml (lint → test matrix → build) and open a PR to watch it run as required status checks.

Duration: ~20 minutes Goal: A working CI workflow running on every PR as required status checks.

Setup

# bash / macOS / Linux / Git Bash
BRANCH="feature/$(whoami)/ci-workflow"
git checkout -b "$BRANCH"
mkdir -p .github/workflows
# Windows PowerShell
$BRANCH = "feature/$env:USERNAME/ci-workflow"
git checkout -b $BRANCH
New-Item -ItemType Directory -Force -Path .github/workflows | Out-Null

Part A — Build ci.yml (15 min)

Create .github/workflows/ci.yml — lint, test, and build.

name: CI

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

# TODO ①: Set default permissions to read-only at workflow level

jobs:
  # ── Job 1: Lint ────────────────────────────────────────────────────
  lint:
    name: Lint
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4

      # TODO ②: Set up Python 3.11 using actions/setup-python

      - name: Cache pip
        uses: actions/cache@v4
        with:
          path: ~/.cache/pip
          key: ${{ runner.os }}-pip-${{ hashFiles('requirements.txt') }}
          restore-keys: ${{ runner.os }}-pip-

      - name: Install linter
        run: pip install ruff

      - name: Run lint
        run: ruff check src/

  # ── Job 2: Test ────────────────────────────────────────────────────
  test:
    name: Test
    runs-on: ubuntu-latest
    # TODO ③: Add a matrix for python-version: ["3.10", "3.11", "3.12"]

    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4

      # TODO ④: Set up Python from the matrix variable

      - name: Install dependencies
        run: pip install -r requirements.txt

      - name: Run tests
        run: pytest src/ --junitxml=test-results.xml

      # TODO ⑤: Upload test-results.xml as an artifact (always, even on failure)

  # ── Job 3: Build ───────────────────────────────────────────────────
  build:
    name: Build
    # TODO ⑥: Make this job wait for lint AND test to succeed
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4

      - name: Build distribution
        run: |
          pip install build
          python -m build

      - name: Upload dist artifact
        uses: actions/upload-artifact@v4
        with:
          name: dist
          path: dist/

💡 Stuck on a CI TODO? Hints with deep links

Read the linked section first. Open the solution (solutions/02-ci-workflow/) only as a last resort.

# TODO Concept Read this
① Set default permissions to read-only permissions: key at workflow level 10-permissions.md → "The permissions: key"
② Set up Python 3.11 Using a third-party action via uses: core-concepts.md → "Steps"
③ Add a matrix for python versions strategy: matrix: caching-and-matrix.md → "Basic matrix"
④ Use the matrix value in setup-python ${{ matrix.<key> }} reference caching-and-matrix.md → "Basic matrix"
⑤ Upload artifact even on failure actions/upload-artifact + if: always() 03-operations-debugging/artifacts.md → "Upload even on failure"
⑥ Make build wait for lint AND test needs: [lint, test] job-orchestration.md → "needs:"

Part B — Push and validate CI on a PR (5 min)

Push your branch and open a PR so ci.yml runs as required status checks.

git add .github/workflows/ci.yml
git commit -m "ci: add lint, test matrix, and build workflow"
git push -u origin "$BRANCH"

Open a PR and watch the checks:

gh pr create --title "ci: add CI workflow" --body "Adds ci.yml (lint, test matrix, build)."
gh pr checks --watch

Verify on the PR's "Checks" tab:

  • lint runs and passes
  • test (3.10), test (3.11), test (3.12) all run in parallel and pass
  • build waits for lint and test to finish, then runs and passes
  • test-results.xml artifact is attached to the run
  • The PR cannot be merged until all CI checks pass

If a check fails: click the failing job in the PR's Checks tab → expand the failing step to read the logs. Fix locally, commit, push — the PR re-runs CI automatically.

Bonus Challenges

  1. SHA-pin your actions — replace @v4 tags with their full commit SHAs
  2. Add a concurrency group so a new push to the same PR cancels in-flight runs
  3. Add a CODEOWNERS rule that requires platform-team review for any .github/workflows/ change

Solution Reference

→ solutions/02-ci-workflow/.github/workflows/ci.yml