Agenda

1. The problem — "works on my machine"
2. What CI is — catching problems automatically
3. What CD is — shipping automatically
4. How it works — pipelines, jobs, and steps
5. GitHub Actions — CI/CD built into GitHub
6. What you gain — why teams adopt it

"It works on my machine"

You write code. It runs perfectly on your laptop.

Your colleague pulls the changes. It crashes.

The server gets the update. It crashes differently.

What went wrong?

• Different Python version
• Missing dependency
• An environment variable not set
• A file only you have locally

Every "works on my machine" moment is a gap in process.

The manual release nightmare

Before CI/CD, shipping code looked like this:

1. Developer finishes a feature
2. Emails the team: "Ready to test"
3. Tester runs it manually on their machine — different to prod
4. Finds a bug — back to developer
5. Two weeks pass
6. Someone runs a script to deploy — forgets a step
7. Production is down at 11pm

This is expensive, slow, and fragile.
Every step is a chance for human error.

Continuous Integration (CI)

Every time someone pushes code, automatically build it and run the tests.

Continuous — it happens on every push, not once a week before a release.

Integration — it integrates your change with the rest of the codebase and checks that everything still works together.

The goal: find problems immediately, when the fix is small and the context is fresh — not three weeks later when no one remembers what changed.

What CI does

When you push code, CI automatically:

1. Checks out your code on a clean machine (not your laptop)
2. Installs dependencies from scratch
3. Runs the tests
4. Reports pass or fail — in your pull request, in your inbox

Tests pass → the change is safe to review and merge

Tests fail → the problem is caught before it reaches anyone else

Why "clean machine" matters

CI runs on a fresh virtual machine every time.

No leftover files. No special settings. No "I have it installed globally".

If your code works in CI, it works anywhere — because CI has nothing your colleagues don't have.

This is how you permanently retire "works on my machine."

Continuous Delivery (CD)

Once code passes CI, automatically deliver it — no manual steps.

Delivery means publishing or deploying: pushing a package to PyPI, deploying to a web server, publishing documentation.

The insight: if the tests passed on a clean machine, the software is ready. Why wait for a human to press a button?

CI vs CD — the full pipeline

developer pushes code
        │
        ▼
   CI: install & test        ← runs on every push
        │
        │  (if tests pass and this is a release)
        ▼
   CD: build & publish       ← runs automatically
        │
        ▼
   users get the update

Together, CI and CD form a pipeline — a repeatable, automated path from code change to running software.

Three concepts: workflow, job, step

Every CI/CD system organises work the same way:

What a workflow looks like

Workflow: "CI"
│
├── Job: test
│   ├── Step: check out the code
│   ├── Step: install Python and dependencies
│   ├── Step: run the linter
│   └── Step: run the tests
│
└── Job: publish          (only runs if 'test' passed)
    ├── Step: check out the code
    ├── Step: build the package
    └── Step: upload to PyPI

Jobs can run in parallel (faster) or in sequence (when one depends on another).

Triggers — what starts a workflow

Workflows don't run constantly. They are triggered by events:

A typical setup: run tests on every push and PR, publish only on a tagged release.

GitHub Actions

GitHub Actions is CI/CD built directly into GitHub.

• No separate service to sign up for
• Free for public repos, generous free tier for private repos
• Workflows live in your repo — in .github/workflows/
• Every workflow is a YAML file

When you push code, GitHub reads your workflow files and runs them automatically on its servers.

A real (minimal) workflow

name: CI

on:
  push:
  pull_request:

jobs:
  test:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v4        # download the code

      - uses: actions/setup-python@v5    # install Python
        with:
          python-version: '3.12'

      - run: pip install -e ".[dev]"     # install dependencies

      - run: pytest                      # run the tests

Push this file. GitHub runs it automatically on every push and PR.

Reading the results

After a push, GitHub shows a status on the commit and in the PR:

All checks passed
Every job succeeded. Safe to merge.

Some checks failed
At least one job failed. Click through to see which step failed and the exact error output.

The output is the same as running the commands locally — but on a clean machine, with a full log you can share with anyone.

Runners — where jobs run

A runner is the machine that executes a job. GitHub provides hosted runners:

Each job gets a fresh, clean machine — installed, used, and discarded.

You can also host your own runners on your own hardware if you need custom specs or private network access.

The benefits

Confidence
Every merged PR has passed the same test suite on a clean machine. You know the code works.

Speed
Problems are caught in minutes, not at the end of a sprint. The fix is always small.

Consistency
The release process is identical every time. No forgotten steps, no "who knows how to deploy this?"

Visibility
Every build has a log. You can see exactly what ran, when it ran, and what it produced — permanently.

Before and after

Boring deployments are the goal.

Getting started

You don't need to write workflow files from scratch.

Rhiza provides a ready-made CI/CD setup for Python projects:

uvx rhiza init        # creates .rhiza/template.yml
uvx rhiza sync        # writes CI workflow files into your repo

You get: CI on every push, linting, test coverage, automated releases, and a weekly sync to keep everything up to date.

Learn more: https://jebel-quant.github.io/rhiza-education/

Key takeaways

1. CI/CD automates the boring, error-prone parts of software development — testing and shipping.

2. CI runs your tests on a clean machine every time you push. "Works on my machine" becomes "works everywhere."

3. CD ships your software automatically once tests pass. No manual steps, no forgotten procedures.

4. GitHub Actions is CI/CD built into GitHub — workflows are YAML files in your repo.

5. You don't need to build this from scratch. Tools like Rhiza wire it up for you in four commands.