Generate a PDF Invoice from a Web Form with Flask + WeasyPrint

If you already collect invoice details in a simple web form, the next step is usually: generate a clean PDF and download it. This Flask + WeasyPrint setup does exactly that. User submits a form, Flask renders an HTML invoice template, and WeasyPrint converts that HTML into a PDF you can download right away.

Typical use cases

Quick internal invoices (freelance, side projects, small business)
Generate PDFs from structured form data (quotes, receipts, reports)
Create consistent PDFs using HTML/CSS instead of “drawing” PDFs manually

Install

pip install flask weasyprint

Note: WeasyPrint may require OS-level libraries depending on your system (common on Linux). If install fails, check the WeasyPrint docs for your OS.

Improved script (safer, cleaner, returns PDF download)

Changes from the original:

Returns the PDF as a download instead of saving a fixed invoice.pdf on disk
Basic input cleanup + allowlist fields (avoid passing everything blindly into the template)
Uses in-memory PDF (BytesIO)
Optional stylesheet support
Better error handling and production-friendly defaults

from __future__ import annotations

from io import BytesIO
from pathlib import Path
from typing import Dict

from flask import Flask, render_template, request, send_file, abort
from weasyprint import HTML, CSS


app = Flask(__name__)

# Optional: basic request size limit (helps avoid giant form posts)
app.config["MAX_CONTENT_LENGTH"] = 1 * 1024 * 1024  # 1 MB

BASE_DIR = Path(__file__).resolve().parent
INVOICE_CSS = BASE_DIR / "static" / "invoice.css"

# Only accept the fields you expect (avoid dumping all request.form into templates)
ALLOWED_FIELDS = {
    "invoice_number",
    "invoice_date",
    "bill_to_name",
    "bill_to_email",
    "bill_to_address",
    "notes",
    "subtotal",
    "tax",
    "total",
}


def clean_form_data(form) -> Dict[str, str]:
    """
    Keep only allowed keys and do light cleanup.
    You can replace this with WTForms/Pydantic if you want stronger validation.
    """
    data: Dict[str, str] = {}
    for k in ALLOWED_FIELDS:
        if k in form:
            # Strip whitespace and keep it simple
            data[k] = form.get(k, "").strip()

    # Minimal required fields (adjust to your needs)
    if not data.get("invoice_number") or not data.get("bill_to_name"):
        abort(400, description="Missing required fields: invoice_number and bill_to_name")

    return data


@app.route("/", methods=["GET", "POST"])
def index():
    if request.method == "GET":
        return render_template("form.html")

    # POST
    data = clean_form_data(request.form)

    # Render invoice HTML from a template
    # IMPORTANT: do not mark user input as |safe in Jinja templates
    html_str = render_template("invoice.html", data=data)

    # Build WeasyPrint HTML and generate PDF bytes in memory
    # base_url helps resolve relative paths (images, etc.) if needed.
    html = HTML(string=html_str, base_url=str(BASE_DIR))

    stylesheets = []
    if INVOICE_CSS.exists():
        stylesheets.append(CSS(filename=str(INVOICE_CSS)))

    pdf_bytes = html.write_pdf(stylesheets=stylesheets)

    # Nice filename for the download
    filename = f"invoice-{data['invoice_number']}.pdf"

    return send_file(
        BytesIO(pdf_bytes),
        mimetype="application/pdf",
        as_attachment=True,
        download_name=filename,
    )


if __name__ == "__main__":
    # Debug off by default for safety; enable only while developing locally
    app.run(host="127.0.0.1", port=5000, debug=False)

Suggested folder structure

your_project/
  app.py
  templates/
    form.html
    invoice.html
  static/
    invoice.css   (optional)

Minimal templates (examples)

templates/form.html

<!doctype html>
<html>
<head>
  <meta charset="utf-8">
  <title>Invoice Form</title>
</head>
<body>
  <h1>Create Invoice</h1>

  <form method="post">
    <label>Invoice #</label><br>
    <input name="invoice_number" required><br><br>

    <label>Invoice date</label><br>
    <input name="invoice_date" placeholder="2026-02-05"><br><br>

    <label>Bill to name</label><br>
    <input name="bill_to_name" required><br><br>

    <label>Bill to email</label><br>
    <input name="bill_to_email"><br><br>

    <label>Address</label><br>
    <textarea name="bill_to_address"></textarea><br><br>

    <label>Subtotal</label><br>
    <input name="subtotal"><br><br>

    <label>Tax</label><br>
    <input name="tax"><br><br>

    <label>Total</label><br>
    <input name="total"><br><br>

    <label>Notes</label><br>
    <textarea name="notes"></textarea><br><br>

    <button type="submit">Generate PDF</button>
  </form>
</body>
</html>

templates/invoice.html

<!doctype html>
<html>
<head>
  <meta charset="utf-8">
  <title>Invoice {{ data.invoice_number }}</title>
</head>
<body>
  <h1>Invoice</h1>

  <p><strong>Invoice #:</strong> {{ data.invoice_number }}</p>
  <p><strong>Date:</strong> {{ data.invoice_date }}</p>

  <h3>Bill To</h3>
  <p>{{ data.bill_to_name }}</p>
  <p>{{ data.bill_to_email }}</p>
  <p style="white-space: pre-line;">{{ data.bill_to_address }}</p>

  <hr>

  <p><strong>Subtotal:</strong> {{ data.subtotal }}</p>
  <p><strong>Tax:</strong> {{ data.tax }}</p>
  <p><strong>Total:</strong> {{ data.total }}</p>

  <hr>

  <p style="white-space: pre-line;">{{ data.notes }}</p>
</body>
</html>

Run it

python app.py

Open: http://127.0.0.1:5000, fill the form, and you’ll get a PDF download.

Small production notes

Don’t run Flask’s built-in server in production. Use a WSGI server (gunicorn/uwsgi) behind a reverse proxy.
Validate numeric fields (subtotal/tax/total) if you rely on them for calculations.
If you store PDFs, generate unique filenames and keep them outside your code folder.

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