Tutorial: QR in Python + Node (portable pattern for PHP + Lua)¶

Verified status as of March 28, 2026. Runtime note: FastFN auto-installs function-local dependencies from requirements.txt / package.json; host runtimes are required in fastfn dev --native, while fastfn dev depends on a running Docker daemon. This tutorial builds the same function in two runtimes:

/qr (Python, SVG response)
/qr@v2 (Node, PNG response)

The goal is to verify runtime-local dependency installation:

Python installs to functions/qr/.deps
Node installs to functions/qr/v2/node_modules

Prerequisites and scope¶

Development mode (fastfn dev): Docker CLI + daemon running.
Native mode (fastfn dev --native): OpenResty in PATH + host runtimes installed for languages you use.
This tutorial validates dependency auto-install only (requirements.txt / package.json per function).
It does not install host python/node runtimes.
The neutral functions/... layout used here is compatibility-friendly. If you prefer runtime-specific examples, keep them inside dedicated language tabs.

1) Create function folders¶

mkdir -p functions/qr
mkdir -p functions/qr/v2

2) Add per-runtime dependency files¶

Python requirements:

cat > functions/qr/requirements.txt <<'EOF'
qrcode>=7.4
EOF

Node package:

cat > functions/qr/v2/package.json <<'EOF'
{
  "name": "fn-node-qr-v2",
  "version": "1.0.0",
  "private": true,
  "dependencies": {
    "qrcode": "^1.5.4"
  }
}
EOF

3) Add function code¶

Python functions/qr/handler.py:

import io
import qrcode
import qrcode.image.svg


def handler(event):
    query = event.get("query") or {}
    text = query.get("url") or query.get("text") or "https://fastfn.io"

    qr = qrcode.QRCode(
        version=1,
        error_correction=qrcode.constants.ERROR_CORRECT_M,
        box_size=8,
        border=2,
    )
    qr.add_data(text)
    qr.make(fit=True)

    img = qr.make_image(image_factory=qrcode.image.svg.SvgImage)
    buf = io.BytesIO()
    img.save(buf)
    svg = buf.getvalue().decode("utf-8")

    return {
        "status": 200,
        "headers": {
            "Content-Type": "image/svg+xml; charset=utf-8",
            "Cache-Control": "no-store",
        },
        "body": svg,
    }

Node functions/qr/v2/handler.js:

const QRCode = require('qrcode');

exports.handler = async (event) => {
  const query = event.query || {};
  const text = query.url || query.text || 'https://fastfn.io';
  const widthRaw = Number(query.size || 320);
  const width = Number.isFinite(widthRaw) ? Math.max(128, Math.min(1024, Math.floor(widthRaw))) : 320;

  const png = await QRCode.toBuffer(text, {
    type: 'png',
    width,
    margin: 2,
    errorCorrectionLevel: 'M',
  });

  return {
    status: 200,
    headers: {
      'Content-Type': 'image/png',
      'Cache-Control': 'no-store',
    },
    is_base64: true,
    body_base64: png.toString('base64'),
  };
};

4) Add per-function policy¶

Python functions/qr/fn.config.json:

{
  "timeout_ms": 60000,
  "max_concurrency": 4,
  "max_body_bytes": 65536,
  "invoke": {
    "methods": ["GET"],
    "summary": "Python QR generator (SVG)",
    "query": {"text": "https://github.com/misaelzapata/fastfn"},
    "body": ""
  }
}

Node functions/qr/v2/fn.config.json:

{
  "timeout_ms": 60000,
  "max_concurrency": 4,
  "max_body_bytes": 65536,
  "invoke": {
    "methods": ["GET"],
    "summary": "Node QR generator (PNG)",
    "query": {"text": "https://github.com/misaelzapata/fastfn", "size": 320},
    "body": ""
  }
}

5) Validate dependency auto-install and runtime output¶

You can reset local dependency folders:

rm -rf functions/qr/.deps functions/qr/v2/node_modules

Call both routes:

curl -sS 'http://127.0.0.1:8080/qr?text=PythonQR' -o /tmp/qr-python.svg
curl -sS 'http://127.0.0.1:8080/qr@v2?text=NodeQR' -o /tmp/qr-node.png

Validate payload types:

file /tmp/qr-python.svg
file /tmp/qr-node.png

Validate runtime-local dependency directories:

test -d functions/qr/.deps/qrcode && echo python-ok
test -d functions/qr/v2/node_modules/qrcode && echo node-ok

6) What this proves¶

Discovery works on public mapped routes, and also supports versioned compatibility routes like /<name>@<version>.
Python and Node can implement the same endpoint independently, and the same contract pattern extends to PHP and Lua handlers.
Each runtime installs dependencies inside its own function directory.
Both binary content types are served correctly through the same gateway.

Flow Diagram¶

flowchart LR
  A["Client request"] --> B["Route discovery"]
  B --> C["Policy and method validation"]
  C --> D["Runtime handler execution"]
  D --> E["HTTP response + OpenAPI parity"]

Objective¶

Clear scope, expected outcome, and who should use this page.

Validation Checklist¶

Command examples execute with expected status codes
Routes appear in OpenAPI where applicable
References at the end are reachable

Troubleshooting¶

If runtime is down, verify host dependencies and health endpoint
If routes are missing, re-run discovery and check folder layout