Invoice Generator

Step 2: Calculate totals

def calculate_invoice(items, tax_rate=0, discount=0):
    subtotal = sum(item['quantity'] * item['unit_price'] for item in items)
    discount_amount = subtotal * (discount / 100)
    taxable_amount = subtotal - discount_amount
    tax_amount = taxable_amount * (tax_rate / 100)
    total = taxable_amount + tax_amount
    return {
        "subtotal": round(subtotal, 2),
        "discount": round(discount_amount, 2),
        "taxable_amount": round(taxable_amount, 2),
        "tax": round(tax_amount, 2),
        "total": round(total, 2)
    }

Step 3: Generate the invoice PDF

Use reportlab to create a clean, professional PDF:

from reportlab.lib import colors
from reportlab.lib.pagesizes import letter
from reportlab.lib.units import inch
from reportlab.platypus import SimpleDocTemplate, Table, TableStyle, Paragraph, Spacer
from reportlab.lib.styles import getSampleStyleSheet

def generate_invoice(data, output_path="invoice.pdf"):
    doc = SimpleDocTemplate(output_path, pagesize=letter)
    styles = getSampleStyleSheet()
    elements = []

    # Header with company info
    elements.append(Paragraph(data['sender']['name'], styles['Title']))
    elements.append(Paragraph(data['sender']['address'], styles['Normal']))
    elements.append(Spacer(1, 0.3 * inch))

    # Invoice details
    elements.append(Paragraph(f"Invoice #{data['invoice_number']}", styles['Heading2']))
    elements.append(Paragraph(f"Date: {data['date']}", styles['Normal']))
    elements.append(Paragraph(f"Due: {data['due_date']}", styles['Normal']))
    elements.append(Spacer(1, 0.3 * inch))

    # Bill to
    elements.append(Paragraph("Bill To:", styles['Heading3']))
    elements.append(Paragraph(data['recipient']['name'], styles['Normal']))
    elements.append(Paragraph(data['recipient']['address'], styles['Normal']))
    elements.append(Spacer(1, 0.3 * inch))

    # Line items table
    table_data = [['Description', 'Qty', 'Unit Price', 'Amount']]
    currency = data.get('currency', '$')
    for item in data['items']:
        amount = item['quantity'] * item['unit_price']
        table_data.append([
            item['description'],
            str(item['quantity']),
            f"{currency}{item['unit_price']:.2f}",
            f"{currency}{amount:.2f}"
        ])

    # Totals rows
    totals = data['totals']
    table_data.append(['', '', 'Subtotal:', f"{currency}{totals['subtotal']:.2f}"])
    if totals.get('discount', 0) > 0:
        table_data.append(['', '', 'Discount:', f"-{currency}{totals['discount']:.2f}"])
    if totals.get('tax', 0) > 0:
        table_data.append(['', '', f"Tax ({data.get('tax_rate', 0)}%):", f"{currency}{totals['tax']:.2f}"])
    table_data.append(['', '', 'TOTAL:', f"{currency}{totals['total']:.2f}"])

    table = Table(table_data, colWidths=[3.5*inch, 0.8*inch, 1.2*inch, 1.2*inch])
    table.setStyle(TableStyle([
        ('BACKGROUND', (0, 0), (-1, 0), colors.HexColor('#1a1a2e')),
        ('TEXTCOLOR', (0, 0), (-1, 0), colors.white),
        ('FONTNAME', (0, 0), (-1, 0), 'Helvetica-Bold'),
        ('GRID', (0, 0), (-1, -1), 0.5, colors.grey),
        ('ALIGN', (1, 0), (-1, -1), 'RIGHT'),
        ('FONTNAME', (2, -1), (3, -1), 'Helvetica-Bold'),
    ]))
    elements.append(table)

    # Payment terms
    elements.append(Spacer(1, 0.5 * inch))
    if data.get('payment_terms'):
        elements.append(Paragraph(f"Payment Terms: {data['payment_terms']}", styles['Normal']))
    if data.get('notes'):
        elements.append(Paragraph(f"Notes: {data['notes']}", styles['Normal']))

    doc.build(elements)
    return output_path

Step 4: Verify and present the output

After generating the PDF:

1. Confirm the file was created successfully
2. Display a summary of the invoice details
3. Show the file path and size

Examples

Example 1: Simple freelance invoice

User request: "Create an invoice for my client Acme Corp for 40 hours of consulting at $150/hour"

Actions taken:

1. Generate invoice number INV-2025-001
2. Calculate totals: 40 x $150 = $6,000
3. Generate PDF

Output:

Invoice generated: invoice_acme_corp.pdf

  Invoice #: INV-2025-001
  Date: January 15, 2025
  Due: February 14, 2025 (Net 30)

  Bill To: Acme Corp

  Line Items:
    Consulting Services - 40 hrs @ $150.00 = $6,000.00

  Subtotal:  $6,000.00
  Total:     $6,000.00

  File saved: invoice_acme_corp.pdf (42 KB)

Example 2: Multi-item invoice with tax

User request: "Invoice for TechStart Ltd: website design $3,500, SEO setup $1,200, hosting 12 months at $29/mo. Add 20% VAT. Currency EUR."

Actions taken:

1. Build line items with three entries
2. Calculate subtotal, VAT, and total in EUR
3. Generate professional PDF

Output:

Invoice generated: invoice_techstart.pdf

  Invoice #: INV-2025-002
  Currency: EUR

  Line Items:
    Website Design          1 x EUR 3,500.00 = EUR 3,500.00
    SEO Setup               1 x EUR 1,200.00 = EUR 1,200.00
    Web Hosting (12 months) 12 x EUR 29.00   = EUR 348.00

  Subtotal:   EUR 5,048.00
  VAT (20%):  EUR 1,009.60
  Total:      EUR 6,057.60

  File saved: invoice_techstart.pdf (45 KB)

Example 3: Invoice with discount

User request: "Create an invoice for Project Alpha: development $8,000, QA testing $2,000. Give 10% early payment discount. Net 15 terms."

Output:

Invoice generated: invoice_project_alpha.pdf

  Line Items:
    Software Development  1 x $8,000.00 = $8,000.00
    QA Testing            1 x $2,000.00 = $2,000.00

  Subtotal:          $10,000.00
  Discount (10%):    -$1,000.00
  Total:              $9,000.00

  Payment Terms: Net 15 - 10% early payment discount applied

Guidelines

• Always auto-generate an invoice number if the user does not provide one. Use the format INV-YYYY-NNN.
• Default payment terms to Net 30 unless the user specifies otherwise.
• Use the current date as the invoice date if not specified.
• Format currency amounts consistently with two decimal places and the correct currency symbol.
• Calculate tax on the post-discount amount, not the subtotal.
• Install reportlab with pip install reportlab if not available.
• Keep the design clean and professional. Use minimal colors and clear typography.
• Include all legally required information: sender details, recipient details, invoice number, date, line items with prices, and total.