Statement PDFs in Python

Why statements become PDFs

A statement summarises a period that is now closed. Regenerating it later from live data would produce a different document, which defeats the point. That is the difference between a document and a view of a database, and it is why finance and account management keep asking for this.

What a statement has to carry

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

• the opening balance and the closing balance
• every transaction in the period with date, description and signed amount
• a running balance column, so a reader can find where a discrepancy starts
• the account identifier, usually masked
• the statement period, which must not overlap the previous one

The Python implementation

httpx is the client most projects here already have. The render happens in an async endpoint that returns a streaming response, and you stream the response with httpx.stream and yield chunks.

import os, httpx
from fastapi import FastAPI
from fastapi.responses import StreamingResponse

app = FastAPI()

@app.get("/statement/{statement_id}.pdf")
async def statement_pdf(statement_id: str):
    html = render_statement(statement_id)

    client = httpx.AsyncClient(timeout=60.0)
    request = client.build_request(
        "POST",
        "https://api.pdfpipe.xyz/v1/pdf",
        headers={"Authorization": f"Bearer {os.environ['PDFPIPE_KEY']}"},
        json={"html": html, "options": {"format": "A4", "printBackground": True}},
    )
    response = await client.send(request, stream=True)

    # stream=True keeps a long statement off the worker's heap.
    return StreamingResponse(
        response.aiter_bytes(),
        media_type="application/pdf",
        background=response.aclose,
    )

The CSS that makes a statement page correctly

The layout problem specific to this document is that a long transaction table spanning many pages, needing repeated headers and a running balance that stays legible after a break. 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 Python

requests holds the entire body in memory before you touch it. httpx.stream does not, and on documents past a few megabytes that difference decides whether your worker survives a burst.

What people try first

Most Python projects reach for WeasyPrint, which is excellent at CSS Paged Media but needs Cairo, Pango and GDK-PixBuf present in the image, and diverges from a browser on modern layout. 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 statement lives afterwards

Rendering is the short part. Statements are the document most often requested again years later, usually for a loan application or an audit. Regenerating one from current data would produce different figures, so the rendered file is the record and has to be stored as issued.

Getting the document right

• Check the closing balance, which has to reconcile with the transactions listed above it.
• Generation is triggered by the end of a billing or accounting period, 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 Python documents, and the same statement in other stacks.