Packing slip PDFs in Flask

Why packing slips become PDFs

A packing slip is printed and physically placed in a box. It exists to be paper. That is the difference between a document and a view of a database, and it is why warehouse and fulfilment teams keep asking for this.

What a packing slip has to carry

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

• the order reference, which the picker checks against the pick list
• each line with SKU, description and quantity shipped
• backordered lines shown explicitly rather than omitted
• the ship-to address as it will appear on the carton
• the carrier reference or tracking number where it exists

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("/packing-slips/<int:packing_slip_id>.pdf")
def packing_slip_pdf(packing_slip_id):
    packing_slip = PackingSlip.query.get_or_404(packing_slip_id)

    html = render_template("packing_slip_pdf.html", packing_slip=packing_slip)

    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"packing-slip-{packing_slip_id}.pdf",
    )

The CSS that makes a packing slip page correctly

The layout problem specific to this document is that it must print correctly on the label and A5 stock warehouses actually use, not just on A4. These rules handle it.

/* Warehouses print on A5 and label stock, not A4. */
@page {
  size: A5;
  margin: 8mm;
}

.signature {
  break-inside: avoid;
  margin-top: auto;
}

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 packing slip lives afterwards

Rendering is the short part. The packing slip is thrown away when the box is opened. It has essentially no retention requirement, which is unusual on this list and worth knowing: the document does not need to be stored, only printed correctly the first time.

Getting the document right

• Check the order reference and quantities, which a picker checks against the physical goods.
• Generation is triggered by an order moving to fulfilment, 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 packing slip in other stacks.