Invoice PDFs in Flask

Why invoices become PDFs

An invoice is a legal record of a demand for payment, and in most jurisdictions it has to be retained in a form that cannot change after issue. A page that re-renders from live data is not that. That is the difference between a document and a view of a database, and it is why finance teams and anyone billing customers keep asking for this.

What a invoice has to carry

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

• an invoice number that is sequential and never reused
• the issue date, and separately the payment due date
• supplier and customer legal names with addresses, not trading names
• tax registration numbers for both parties where the jurisdiction requires them
• per-line quantity, unit price, tax rate and line total
• the tax subtotal broken out by rate, then the gross total

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("/invoices/<int:invoice_id>.pdf")
def invoice_pdf(invoice_id):
    invoice = Invoice.query.get_or_404(invoice_id)

    html = render_template("invoice_pdf.html", invoice=invoice)

    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"invoice-{invoice_id}.pdf",
    )

The CSS that makes a invoice page correctly

The layout problem specific to this document is that line items are unbounded, so the table has to break across pages while the totals block stays whole and the column headers repeat. These rules handle it.

/* Line items run past one page. Repeat the header, keep the
   totals block whole, and never strand a single row. */
thead { display: table-header-group; }
tfoot { display: table-footer-group; }
tr    { break-inside: avoid; }

.totals {
  break-inside: avoid;
  break-before: auto;
}

@page {
  size: A4;
  margin: 18mm 16mm 22mm;
}

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 invoice lives afterwards

Rendering is the short part. Invoices are kept for the statutory retention period, which in most jurisdictions runs to six years or more. That means the file has to be stored, not regenerated, and the stored copy has to be the one that was sent.

Getting the document right

• Check the total. A rounding difference between what you display and what you charge is the kind of error that surfaces during an audit rather than on the day.
• Generation is triggered by an order completing, or a billing cycle closing, 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 invoice in other stacks.