Packing slip PDFs in Python

Why packing slips become PDFs

A packing slip is printed and physically placed in a box. It exists to be paper. That is the difference between a document and a view of a database, and it is why warehouse and fulfilment teams keep asking for this.

What a packing slip has to carry

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

• the order reference, which the picker checks against the pick list
• each line with SKU, description and quantity shipped
• backordered lines shown explicitly rather than omitted
• the ship-to address as it will appear on the carton
• the carrier reference or tracking number where it exists

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("/packing-slip/{packing_slip_id}.pdf")
async def packing_slip_pdf(packing_slip_id: str):
    html = render_packing_slip(packing_slip_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 packing slip off the worker's heap.
    return StreamingResponse(
        response.aiter_bytes(),
        media_type="application/pdf",
        background=response.aclose,
    )

The CSS that makes a packing slip page correctly

The layout problem specific to this document is that it must print correctly on the label and A5 stock warehouses actually use, not just on A4. These rules handle it.

/* Warehouses print on A5 and label stock, not A4. */
@page {
  size: A5;
  margin: 8mm;
}

.signature {
  break-inside: avoid;
  margin-top: auto;
}

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 packing slip lives afterwards

Rendering is the short part. The packing slip is thrown away when the box is opened. It has essentially no retention requirement, which is unusual on this list and worth knowing: the document does not need to be stored, only printed correctly the first time.

Getting the document right

• Check the order reference and quantities, which a picker checks against the physical goods.
• Generation is triggered by an order moving to fulfilment, 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 Python documents, and the same packing slip in other stacks.