Ticket PDFs in Flask

Why tickets become PDFs

A ticket has to work at a gate with no signal, on a phone that may be offline, or on paper. Anything requiring a live request fails at exactly the wrong moment. That is the difference between a document and a view of a database, and it is why event organisers and travel operators keep asking for this.

What a ticket has to carry

Before any of the code below matters, the template has to produce a document that is actually a ticket. These are the fields that make it one, and the ones a reader or an auditor will look for first.

• the scannable code, which is the ticket; everything else is decoration
• the event or journey, with date and time including timezone
• the seat, zone or class where the entitlement is specific
• the holder name where the ticket is non-transferable
• the terms of entry, which are what a gate refusal is justified by

Turning your Jinja2 template into a document

You already have the markup. Render_template("invoice.html", **context) gives you it as a string, which is the only input the render needs. url_for("static", filename=...) produces a path, not a URL. Pass _external=True when the markup is going to be rendered somewhere other than the user's browser, or inline the stylesheet.

The Flask implementation

httpx is the client most projects here already have. The render happens in a route returning send_file, and you return send_file with a BytesIO and mimetype="application/pdf".

# app.py
from io import BytesIO

import httpx
from flask import render_template, send_file


@app.get("/tickets/<int:ticket_id>.pdf")
def ticket_pdf(ticket_id):
    ticket = Ticket.query.get_or_404(ticket_id)

    html = render_template("ticket_pdf.html", ticket=ticket)

    upstream = httpx.post(
        "https://api.pdfpipe.xyz/v1/pdf",
        headers={"Authorization": f"Bearer {app.config['PDFPIPE_KEY']}"},
        json={"html": html, "options": {"format": "A4", "printBackground": True}},
        timeout=60.0,
    )
    upstream.raise_for_status()

    return send_file(
        BytesIO(upstream.content),
        mimetype="application/pdf",
        download_name=f"ticket-{ticket_id}.pdf",
    )

The CSS that makes a ticket page correctly

The layout problem specific to this document is that a barcode or QR code must render at a size and contrast a scanner can actually read after printing. These rules handle it.

/* The code has to survive printing. Keep it black on white at a
   size a scanner can resolve, and never let it break across a page. */
.barcode {
  break-inside: avoid;
  image-rendering: pixelated;
  min-width: 45mm;
  filter: none;
}

@page { size: A6 landscape; margin: 5mm; }

What goes wrong in Flask

The development server handles one request at a time by default, so a render that takes two seconds makes the whole app look dead for two seconds. That is a dev-server artefact rather than a real problem, but it is the reason people conclude PDF generation is slow before they have deployed anything.

What people try first

Most Flask projects reach for pdfkit, which is a wrapper around the wkhtmltopdf binary. The binary has to be present on every machine, and its rendering engine is a fork of WebKit that stopped tracking the web around 2016. That works until it is running on more than one machine, at which point the browser becomes the thing you operate rather than the thing you use.

Where the ticket lives afterwards

Rendering is the short part. A ticket is worthless the moment the event ends, so nothing needs to survive it except the record that it was scanned. That record lives in your database, not in the PDF.

Getting the document right

• Check the code itself. A ticket that will not scan is worthless no matter how it looks.
• Generation is triggered by a booking being confirmed, so size the timeout for that path rather than for a health check.
• Volume arrives in bursts, so queue the render rather than doing it inside the request that triggered it.
• Backgrounds are painted by default here, so a design that uses colour needs nothing set. Only an explicit print_background of false turns them off.

Frequently asked

Related

Other Flask documents, and the same ticket in other stacks.