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Otto
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Build a Simple Invoice Generator in Python

#python #tutorial #freelancing #productivity

Build a Simple Invoice Generator in Python

If you freelance or run a small business, generating invoices manually is a waste of time.

In this tutorial, I will show you how to build a clean invoice generator in Python — completely from scratch.

What We Are Building

  • Read invoice data from a JSON file
  • Calculate subtotals, VAT, and total
  • Output a formatted text invoice

Step 1: Invoice Data Structure

Create invoice_data.json:

{
  "invoice_number": "INV-2026-001",
  "date": "2026-03-22",
  "due_date": "2026-04-22",
  "client": {"name": "Acme Corp", "email": "billing@acme.com"},
  "freelancer": {"name": "Your Name", "email": "you@example.com"},
  "items": [
    {"description": "Web Development", "quantity": 20, "unit_price": 75},
    {"description": "SEO Audit", "quantity": 1, "unit_price": 350}
  ],
  "vat_rate": 0.20,
  "currency": "EUR"
}
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Step 2: The Generator Script 

import json

def load_invoice(filepath):
    with open(filepath) as f:
        return json.load(f)

def calculate_totals(items, vat_rate):
    subtotal = sum(item["quantity"] * item["unit_price"] for item in items)
    vat = subtotal * vat_rate
    return subtotal, vat, subtotal + vat

def format_invoice(data):
    subtotal, vat, total = calculate_totals(data["items"], data["vat_rate"])
    cur = data.get("currency", "EUR")
    lines = [
        "=" * 60,
        f"INVOICE {data["invoice_number"]}",
        f"Date: {data["date"]} | Due: {data["due_date"]}",
        "=" * 60,
    ]
    for item in data["items"]:
        line_total = item["quantity"] * item["unit_price"]
        lines.append(f"{item["description"]:<35} {item["quantity"]:>3} x {item["unit_price"]:>8.2f} = {line_total:>8.2f} {cur}")
    lines += [
        "-" * 60,
        f"Subtotal: {subtotal:.2f} {cur}",
        f"VAT ({data["vat_rate"]:.0%}): {vat:.2f} {cur}",
        f"TOTAL: {total:.2f} {cur}",
        "=" * 60,
    ]
    return "\n".join(lines)

if __name__ == "__main__":
    import sys
    data = load_invoice(sys.argv[1] if len(sys.argv) > 1 else "invoice_data.json")
    print(format_invoice(data))
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Step 3: Run It 

python invoice_generator.py invoice_data.json
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The script outputs a clean, formatted invoice with automatic VAT calculation.

Extending the Script

  • PDF export with reportlab
  • Email sending with Python built-in smtplib
  • Multi-currency support via a free exchange rate API
  • Batch processing for multiple clients

Why Build Your Own?

  • 100% private — no data sent anywhere
  • Customizable to your country tax rules
  • Works offline
  • Zero subscription fees

Ready-Made Version

I packaged a polished version with FR/EU VAT, IBAN, multiple templates, and client management into Freelancer OS on Gumroad — complete toolkit for freelancers, EUR 19.

Building this? Drop questions in the comments.

Full freelance toolkit: guittet.gumroad.com

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