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How to create a PDF Invoice Generator app with Python

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Introduction

As I subscribed to daily python projects, so I receive a project daily from the website. Today I received a project: create a PDF Invoice Generator with Python. And I think it’s interesting enough for me to get into. There are some requirements for the app

  • The app is a command-line tool that generates PDF invoices for customers.
  • The user provides customer information, purchased items, and prices through a JSON input file. The tool calculates totals, applies taxes, and saves the invoice as a PDF document.

The PDF will look like: https://dailypythonprojects.substack.com/api/v1/file/e476b8b3-f598-48cf-a2cb-07a28ac8c6a3.pdf

image.png

Requirements

How the app works

The app loads data from invoice_data.json and then added data to cells in tables (business name, business address, customer name, customer address, items…). Once the user runs the app, it will generate to a pdf file from invoice_data.json.

Steps to build the app

1. Create a generate_json.py

This is a tool to create a json file from a sample json provided. Here is a sample json file:

{
  "business_name": "Tech Solutions Inc.",
  "business_address": "123 Innovation Drive, Tech City, TX 75001",
  "customer_name": "Jane Doe",
  "customer_address": "456 Elm Street, Springfield, IL 62704",
  "items": [
    {"description": "Laptop", "quantity": 1, "unit_price": 1000.00},
    {"description": "Mouse", "quantity": 2, "unit_price": 25.50},
    {"description": "Keyboard", "quantity": 1, "unit_price": 49.99}
  ],
  "tax_rate": 0.07
}

I want to generate not just 3 items, I want to test a pdf invoice generator with around 100 items.

import json
import random

I import 2 libraries: json and random from python.

And then I created an array of product names. Because the items in sample json file are only 3 computer accessories. For a pdf invoice with around 100 items. I want to have at least 20 items.

Product name string array:

product_names = [
    "Laptop", "Mouse", "Keyboard", "Monitor", "Printer", "Tablet", "Router", 
    "Webcam", "Headset", "Smartphone", "Speaker", "Hard Drive", "Memory Stick", 
    "Charger", "Graphics Card", "Processor", "Motherboard", "Cooling Fan", 
    "Power Supply", "Network Switch", "USB Hub", "External SSD", "Microphone", 
    "Projector", "Stylus Pen"
]

JSON structure:

data_with_random_names = {
    "business_name": "Tech Solutions Inc.",
    "business_address": "123 Innovation Drive, Tech City, TX 75001",
    "customer_name": "Jane Doe",
    "customer_address": "456 Elm Street, Springfield, IL 62704",
    "items": [],
    "tax_rate": 0.07
}

These data can be modified directly from json file.

Generate 100 random items with random names from the product list.

  • Description: random names from product_names array
  • Quantity: random numbers from 1 to 10
  • Unit price: random floats with 2 digits after the dot
for i in range(1, 101):
    item = {
        "description": random.choice(product_names),
        "quantity": random.randint(1, 10),  # Random quantity between 1 and 10
        "unit_price": round(random.uniform(5.0, 500.0), 2)  # Random price between $5 and $500
    }
    data_with_random_names["items"].append(item)

Save the JSON data to a file and print file name on the screen.

output_file = "invoice_data.json"
with open(output_file, "w") as file:
    json.dump(data_with_random_names, file, indent=2)

print(f"JSON file with random product names saved as {output_file}")

In the terminal:

run python .\generate_json.py

to create an invoice_data.json file

2. Create an invoice_generator.py

Libraries:

import json
from fpdf import FPDF
import argparse

FPDF is a library used for generating pdf files. For more information, check it out at this website: https://pyfpdf.readthedocs.io/

First, we need to create InvoiceGenerator class

class InvoiceGenerator(FPDF):

By passing FPDF, the InvoiceGenerator class inherits all the methods and properties from FPDF.

Inside InvoiceGenertor(FPDF):

Create Header method:

def header(self):
        self.set_y(5)
        self.set_font('Arial', 'B', 16)
        self.cell(0, 10, 'INVOICE', border = 0, ln = 1, align='C')
        self.ln(10)

Set value of ordinate 5

Set font: Arial, bold, size 16

Set Cell: width 0, height: 10, text=”Invoice”, border =0, ln = 1 to the beginning of the next line, align center.

Add line break .ln(10)

As an invoice, it should have Header, Business information, Customer information, items table with calculation subtotal, tax, total, and footer.

Footer:

def footer(self):
        # Add the footer with page numbers
        self.set_y(-10)
        self.set_font('Arial', 'I', 8)
        self.cell(0, 10, f'Page {self.page_no()}', align='C')

f'Page python {self.page_no()} ' is a formatted string literal. f’….’ alows embed expression inside a string enclosed in curly braces . Inside curly braces: accessing page_no() method of FPDF, it returns the current page of the PDF file.

Items table

def add_items_table(self, items, tax_rate):
        
        #add table header
        self.set_font('Arial', 'B', 10)
        self.cell(80, 10, 'Description', border = 1)
        self.cell(30, 10, 'Quantity', border = 1, align = 'C')
        self.cell(40, 10, 'Unit Price', border = 1, align = 'C')
        self.cell(40, 10, 'Total', border = 1, align = 'C')
        self.ln()

        #Add items into the table
        self.set_font('Arial', '', 10)
        subtotal = 0

        for item in items:
            total = round(item['quantity'] * item['unit_price'], 2)
            self.cell(80, 10, item['description'], border = 1)
            self.cell(30, 10, str(item['quantity']), border = 1, align = 'C')
            self.cell(40, 10, str(item['unit_price']), border=1, align = 'C')
            self.cell(40, 10, str(total), border = 1, align = 'C')
            self.ln()        
            subtotal += total
        
        #calculate Tax and total
        tax = subtotal * tax_rate
        grand_total = subtotal + tax

        #Add subtotal, tax and total
        self.ln(5)
        self.set_font('Arial', 'B', 10)
        

        self.cell(150, 10, 'Subtotal', border=1, align='R')
        self.cell(40, 10, f'${subtotal:.2f}', border=1, align='C')
        self.ln()
        self.cell(150, 10, 'Tax', border=1, align='R')
        self.cell(40, 10, f'${tax:.2f}', border=1, align='C')
        self.ln()
        self.cell(150, 10, 'Total', border=1, align='R')
        self.cell(40, 10, f'${grand_total:.2f}', border=1, align='C')

The add_items_table uses 2 params: items and tax_rate, which are items and tax_rate fields in JSON file.

Inside the method, use a for loop to loop all items in items array.

Each item, I implemented a calculation total: quantity * unit price, and then added it to subtotal.

After looping all items, in this case 100 items, I need to calculate the tax and grand_total with tax.

  • Create a generate_invoice(json_file, pdf_output_ile) to handle PDF file creation.

The method takes 2 params json_file, and pdf_output_file

  • It reads and parses the JSON data into a variable data.
  • It creates an instance of InvoiceGenerator class and then call methods inside the class.
    • Add_page(): adds new page to the PDF. This is a method from FPDF
    • Add_business(…), add_customer_info(…), add_items_table(…)
    • Output(pdf_output_file, ‘F’): this is a method from FPDF, pdf_output_file is the name of the file, ‘F’ indicates saving to a local file with the given name.

I use argparse library to handle command-line arguments:

  • json_file: positional argument, it’s mandatory, and it specifies the path to the JSON input file.
  • -o, —output is optional argument: it specifies the name of output PDF, default is ‘Invoice.pdf’

I use name = ‘main’ to make sure the script executing the code only when it is run directly.

3. Run the app

In terminal, run the command like:

python Invoice_Generator.py invoice_data.json -o Invoice.pdf

4. Source code

Github Link

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