Payslip PDFs in Flask

Why payslips become PDFs

A payslip is a personal financial record an employee keeps, uses to apply for credit, and expects to be able to open years later. That is the difference between a document and a view of a database, and it is why HR and payroll teams keep asking for this.

What a payslip has to carry

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

• gross pay, itemised by basic, overtime and any allowances
• each deduction named separately with its own amount
• net pay, which is the number the employee checks first
• year-to-date totals for gross, tax and net
• the pay period and the payment date, which are not the same day
• the employee identifier and tax reference

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("/payslips/<int:payslip_id>.pdf")
def payslip_pdf(payslip_id):
    payslip = Payslip.query.get_or_404(payslip_id)

    html = render_template("payslip_pdf.html", payslip=payslip)

    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"payslip-{payslip_id}.pdf",
    )

The CSS that makes a payslip page correctly

The layout problem specific to this document is that gross, deductions and net must be visible together without scrolling or breaking, on one page, every time. These rules handle it.

/* Gross, deductions and net have to be visible together. One page,
   every time, regardless of how many deduction lines there are. */
.payslip { break-inside: avoid; }
.summary { break-inside: avoid; }

@page { size: A4; margin: 15mm; }

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 payslip lives afterwards

Rendering is the short part. Payslips are personal financial records the employee will need for credit applications and tax filings for years. Employers commonly retain them for six years, and employees expect to be able to download an old one long after they have left.

Getting the document right

• Check net pay and the deduction breakdown, which is what an employee checks first and complains about immediately.
• Generation is triggered by a payroll run, producing every employee's document at once, so size the timeout for that path rather than for a health check.
• This is a batch shape: render the run through the batch endpoint rather than firing thousands of individual requests.
• 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 payslip in other stacks.