Contract PDFs in FastAPI

Why contracts become PDFs

A contract has to be fixed at signature. Both parties need to be certain they are looking at identical words, which a rendered page cannot guarantee. That is the difference between a document and a view of a database, and it is why legal and sales operations keep asking for this.

What a contract has to carry

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

• the parties, by legal entity name and registered number
• the effective date and the term
• numbered clauses with stable numbering, since amendments reference them
• the commercial schedule, usually as an annexe rather than in the body
• signature blocks with name, title and date for each party

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/contracts.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("/contracts/{contract_id}.pdf")
async def contract_pdf(contract_id: int, db=Depends(get_db)):
    contract = await db.get_contract(contract_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("contract.html").render(contract=contract)

    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 contract page correctly

The layout problem specific to this document is that numbered clauses with cross-references, where a page break in the wrong place separates a clause from the term it defines. These rules handle it.

/* A clause must not be separated from the term it defines, and a
   signature block alone on a final page invites a dispute. */
.clause     { break-inside: avoid; }
.signatures { break-inside: avoid; break-before: avoid; }

p { orphans: 3; widows: 3; }

@page { size: A4; margin: 25mm 20mm; }

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 contract lives afterwards

Rendering is the short part. A contract is retained for the term plus the limitation period, which typically means well over a decade. Immutability matters more here than anywhere else on this list: both parties must be able to prove the words have not moved.

Getting the document right

• Check the parties and the effective date, which is what every later dispute turns on.
• Generation is triggered by agreement being reached, often with a signature workflow following, so size the timeout for that path rather than for a health check.
• These are produced one at a time by a person who is waiting, so latency is felt directly.
• 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 contract in other stacks.