Payslip PDFs in FastAPI

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 Jinja2Templates template into a document

You already have the markup. Templates.get_template("invoice.html").render(context) gives you it as a string, which is the only input the render needs. TemplateResponse is for returning HTML to a browser. When the HTML is going to a renderer you want the string, so call the underlying Jinja template directly rather than unwrapping a Response you never send.

The FastAPI implementation

httpx is the client most projects here already have. The render happens in an async endpoint returning StreamingResponse, and you return StreamingResponse over the httpx byte iterator.

# routers/payslips.py
import httpx
from fastapi import APIRouter, Depends
from fastapi.responses import StreamingResponse
from fastapi.templating import Jinja2Templates

router = APIRouter()
templates = Jinja2Templates(directory="templates")


@router.get("/payslips/{payslip_id}.pdf")
async def payslip_pdf(payslip_id: int, db=Depends(get_db)):
    payslip = await db.get_payslip(payslip_id)

    # The template directly, not TemplateResponse: we want the string, and
    # TemplateResponse is for HTML you are actually sending to a browser.
    html = templates.get_template("payslip.html").render(payslip=payslip)

    client = httpx.AsyncClient(timeout=60.0)
    request = client.build_request(
        "POST",
        "https://api.pdfpipe.xyz/v1/pdf",
        headers={"Authorization": f"Bearer {settings.pdfpipe_key}"},
        json={"html": html, "options": {"format": "A4", "printBackground": True}},
    )
    upstream = await client.send(request, stream=True)
    upstream.raise_for_status()

    return StreamingResponse(
        upstream.aiter_bytes(),
        media_type="application/pdf",
        background=BackgroundTask(upstream.aclose),
    )

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 FastAPI

Calling a synchronous HTTP client inside an async def endpoint blocks the event loop for the whole render, which stalls every other request on that worker. Either use httpx.AsyncClient, or declare the endpoint def rather than async def so it runs in the threadpool.

What people try first

Most FastAPI projects reach for WeasyPrint called from a background task, which moves the memory cost off the request but not out of the process. 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 FastAPI documents, and the same payslip in other stacks.