Docker Python Lab Guide

Formerly the standalone skill docker-python-lab.

What This Guide Does

Generates interactive Python lab blocks for MkDocs pages in the learning-python textbook (or any MkDocs project following the same pattern). Each lab shows a code editor, Run and Reset buttons, and an output area. Code runs inside a fresh, isolated Docker container via a local HTTP service on port 5001.

This is the Docker counterpart to the Skulpt lab pattern — same look and feel, but students run real Python (not browser-emulated Python) with access to the full standard library and third-party packages.

Use it whenever someone asks to add a Python lab, code runner, interactive Python exercise, or runnable code block to a textbook page that uses Docker (not Skulpt). Always follow this guide instead of writing docker lab HTML by hand.

Prerequisites: Shared Infrastructure

Before generating any lab block, make sure the shared files exist. Check once per session; skip if already in place.

1. Check / create `docs/css/docker-lab.css`

If the file is missing, copy from this skill's assets:

cp $BK_HOME/skills/microsim-generator/assets/docker-python-lab/docker-lab.css \
   docs/css/docker-lab.css

2. Check / create `docs/js/docker-lab.js`

cp $BK_HOME/skills/microsim-generator/assets/docker-python-lab/docker-lab.js \
   docs/js/docker-lab.js

3. Add to `mkdocs.yml` (if not already present)

Check extra_css and extra_javascript in mkdocs.yml. If the entries are missing, add them:

extra_css:
  - css/docker-lab.css      # ← add this line

extra_javascript:
  - js/docker-lab.js        # ← add this line

Do not duplicate entries that are already there.

4. Verify the service script exists

The runtime service lives at scripts/run-python-docker.sh. If it is missing from the project, tell the user — the labs won't work without it.

Lab HTML Template

Each lab needs a unique suffix — a short string that distinguishes it from other labs on the same page. Use "1" for the first lab, "2" for the second, and so on. The suffix appears in every element ID so the JS can find the right elements.

Standard lab (text output only)

<div id="docker-lab-SUFFIX">
<div id="docker-editor-SUFFIX">
<textarea id="docker-code-SUFFIX" spellcheck="false">PYTHON_CODE_HERE
</textarea>
<div id="docker-buttons-SUFFIX">
  <button id="docker-run-SUFFIX" onclick="runDocker('SUFFIX')">&#9654; Run</button>
  <button id="docker-reset-SUFFIX" onclick="resetDocker('SUFFIX')">&#8635; Reset</button>
</div>
<pre id="docker-output-SUFFIX" class="docker-output">Output will appear here after you click Run.</pre>
</div>
</div>

Rules:

Replace every SUFFIX with the same string (e.g., 1, 2, hello-world).
Put the Python code directly inside the <textarea> tag — no extra indentation relative to column 0, because leading spaces become part of the code.
The </textarea> closing tag must be on its own line with no trailing spaces before it; any blank line between the code and </textarea> adds an empty line in the editor.
Do not add rows="N" — the JS auto-sizes the textarea to fit the code.
Do not add id="main" to any element — that conflicts with the p5.js canvas parent convention used elsewhere in this project.

Timed lab (shows phase-by-phase timing breakdown)

Only add this variant when the lesson is specifically about how Docker execution works. It requires suffix "4" by convention and adds a timing table below the buttons.

<div id="docker-lab-4">
<div id="docker-editor-4">
<textarea id="docker-code-4" spellcheck="false">PYTHON_CODE_HERE
</textarea>
<div id="docker-buttons-4">
  <button id="docker-run-4" onclick="runDockerTimed()">&#9654; Run + Time</button>
  <button id="docker-reset-4" onclick="resetDockerTimed()">&#8635; Reset</button>
</div>
</div>
<div id="docker-timing-display" style="display:none; margin-top:12px;">
  <table id="docker-timing-table">
    <thead>
      <tr><th>#</th><th>Phase</th><th style="text-align:right">Time (ms)</th><th>Bar</th></tr>
    </thead>
    <tbody>
      <tr><td>1</td><td>Send to service (network)</td>
          <td id="td-network-send" style="text-align:right">—</td>
          <td><div class="timing-bar" id="bar-network-send"></div></td></tr>
      <tr><td>2</td><td>Container startup</td>
          <td id="td-startup" style="text-align:right">—</td>
          <td><div class="timing-bar" id="bar-startup"></div></td></tr>
      <tr><td>3</td><td>Python execution</td>
          <td id="td-exec" style="text-align:right">—</td>
          <td><div class="timing-bar" id="bar-exec"></div></td></tr>
      <tr><td>4</td><td>Return to browser (network)</td>
          <td id="td-network-return" style="text-align:right">—</td>
          <td><div class="timing-bar" id="bar-network-return"></div></td></tr>
      <tr style="font-weight:bold; border-top: 2px solid #642580;">
          <td colspan="2">Total round-trip</td>
          <td id="td-total" style="text-align:right">—</td>
          <td></td></tr>
    </tbody>
  </table>
  <p id="docker-timing-note" style="font-size:0.85em; color:#666; margin-top:6px;"></p>
</div>
<pre id="docker-output-4" class="docker-output" style="margin-top:10px;">Output will appear here after you click Run + Time.</pre>
</div>

Complete Page Pattern

A page with three labs looks like this:

# Page Title

Intro text explaining what the page covers and that students must run
`bash scripts/run-python-docker.sh` in a separate terminal first.

## Lab 1 — Hello, World!

Brief explanation of what this lab demonstrates.

<div id="docker-lab-1">
<div id="docker-editor-1">
<textarea id="docker-code-1" spellcheck="false">print("Hello, World!")
</textarea>
<div id="docker-buttons-1">
  <button id="docker-run-1" onclick="runDocker('1')">&#9654; Run</button>
  <button id="docker-reset-1" onclick="resetDocker('1')">&#8635; Reset</button>
</div>
<pre id="docker-output-1" class="docker-output">Output will appear here after you click Run.</pre>
</div>
</div>

**Try these experiments:**
- Change the message and run again.

---

## Lab 2 — Variables

<div id="docker-lab-2">
<div id="docker-editor-2">
<textarea id="docker-code-2" spellcheck="false">x = 42
print("The answer is", x)
</textarea>
<div id="docker-buttons-2">
  <button id="docker-run-2" onclick="runDocker('2')">&#9654; Run</button>
  <button id="docker-reset-2" onclick="resetDocker('2')">&#8635; Reset</button>
</div>
<pre id="docker-output-2" class="docker-output">Output will appear here after you click Run.</pre>
</div>
</div>

Choosing Good Lab Code

Keep starter code short (5–15 lines) so students can read it at a glance.
Every lab should produce visible output — at least one print() call.
Avoid input() — there is no stdin in the Docker runner.
Avoid file I/O — the container has no persistent filesystem.
Avoid import of non-stdlib packages unless python:3.11-alpine includes them (it ships only the stdlib).
Match the "See It — Run It — Modify It" rhythm from the Skulpt labs: starter code runs and shows something, then "Try these experiments" gives specific, achievable modifications.

Service Not Running — User-Facing Error

If a student clicks Run before starting the service, the lab shows:

Cannot connect to the Python Docker service. Please open a terminal and run: bash scripts/run-python-docker.sh Then reload this page and try again.

No extra error handling is needed in the page markdown — docker-lab.js handles this automatically.

Multiple Labs per Page — Checklist

When generating labs for a single page:

Each lab has a unique suffix — 1, 2, 3, …
Every element ID includes that suffix (docker-lab-1, docker-code-1, etc.)
Button onclick passes the matching suffix string: runDocker('2')
No two labs share a suffix on the same page
The page opens with a "Start the Docker service" instruction block
docs/css/docker-lab.css and docs/js/docker-lab.js are in mkdocs.yml

/microsim-generator

referencesdocker-python-lab-guide.md