Invoice Generator

A Python-based invoice management application that allows you to create invoices, calculate totals, store invoice data in SQLite, and generate professional PDF invoices.

The project is designed as a practical Python application with separate modules for invoice calculations, database management, PDF generation, and the command-line interface.

Features

Create new invoices
Add multiple products or services to an invoice
Calculate item totals automatically
Calculate invoice subtotal
Calculate tax
Apply discounts
Calculate the final invoice total
Generate unique invoice numbers
Save invoices to a SQLite database
Store invoice items separately from invoice information
View previously saved invoices
List invoice history
Delete invoices
Generate PDF invoices
Automatically create required directories
Input validation for numbers and required fields
Command-line interface

Project Structure

invoice-generator/
│
├── main.py
├── invoice.py
├── models.py
├── database.py
├── pdf_generator.py
│
├── data/
│   └── invoices.db
│
└── invoices/
    ├── INV-0001.pdf
    ├── INV-0002.pdf
    └── ...

File Responsibilities

`main.py`

The main entry point of the application.

It handles:

Application startup
Database initialization
User input
Main menu
Creating invoices
Listing invoices
Viewing invoices
Deleting invoices

`invoice.py`

Contains the invoice calculation logic.

It handles:

Creating invoice data
Generating invoice numbers
Calculating item totals
Calculating subtotal
Calculating tax
Calculating discounts
Calculating the final total

Example:

subtotal = 500

tax = subtotal * (14 / 100)

total = subtotal + tax - discount

`models.py`

Contains the data models used by the application.

The project uses Python dataclasses to represent:

Invoice items
Invoices

Example:

InvoiceItem(
    name="Website",
    quantity=1,
    unit_price=500
)

An invoice can contain multiple InvoiceItem objects.

`database.py`

Handles SQLite database operations.

It is responsible for:

Creating the database
Creating database tables
Saving invoices
Saving invoice items
Retrieving invoices
Listing invoices
Deleting invoices

The database is stored at:

data/invoices.db

`pdf_generator.py`

Handles PDF generation using ReportLab.

It creates PDF invoices containing:

Invoice number
Date
Customer information
Invoice items
Quantities
Unit prices
Item totals
Subtotal
Tax
Discount
Final total

Generated PDFs are stored in:

invoices/

How the Application Works

The application follows a simple flow:

                    ┌───────────────┐
                    │    main.py    │
                    │  User Input   │
                    └───────┬───────┘
                            │
                            ▼
                    ┌───────────────┐
                    │  invoice.py   │
                    │ Calculations  │
                    └───────┬───────┘
                            │
                 ┌──────────┴──────────┐
                 │                     │
                 ▼                     ▼
        ┌─────────────────┐   ┌──────────────────┐
        │   database.py   │   │ pdf_generator.py │
        │     SQLite      │   │      PDF         │
        └────────┬────────┘   └────────┬─────────┘
                 │                     │
                 ▼                     ▼
        ┌─────────────────┐   ┌──────────────────┐
        │ data/invoices.db│   │ invoices/*.pdf   │
        └─────────────────┘   └──────────────────┘

Requirements

You need:

Python 3.10 or newer
pip
ReportLab

SQLite is included with Python, so no separate SQLite installation is required.

Installation

1. Clone the repository

git clone https://github.com/yourusername/invoice-generator.git

Move into the project:

cd invoice-generator

2. Create a virtual environment

Linux/macOS:

python3 -m venv venv

Windows:

python -m venv venv

3. Activate the virtual environment

Linux/macOS

source venv/bin/activate

Windows

venv\Scripts\activate

4. Install dependencies

pip install reportlab

Running the Application

Run:

python main.py

On Linux systems where python points to Python 2 or is unavailable:

python3 main.py

You should see:

============================================================
                   INVOICE GENERATOR
============================================================

1. Create new invoice
2. List invoices
3. View invoice
4. Delete invoice
5. Exit

Choose an option:

Creating an Invoice

Select:

1. Create new invoice

The application asks for customer information:

Customer name: John Smith
Customer email: john@example.com

Then you can add products or services:

--- Invoice Items ---

Add Item

Item/service name: Website Development
Quantity: 1
Unit price: 500

Add another item? (y/n): y

Add Item

Item/service name: Hosting
Quantity: 1
Unit price: 50

Add another item? (y/n): n

Then enter the tax and discount:

Tax (%): 14
Discount: 20

The application calculates the invoice automatically.

Invoice Calculation

Suppose an invoice contains:

Website Development
Quantity: 1
Unit Price: $500

Hosting
Quantity: 1
Unit Price: $50

The item totals are:

Website Development = 1 × $500 = $500

Hosting = 1 × $50 = $50

The subtotal becomes:

Subtotal = $500 + $50

Subtotal = $550

If the tax rate is 14%:

Tax = $550 × 14 / 100

Tax = $77

If the discount is $20:

Total = Subtotal + Tax - Discount

Total = $550 + $77 - $20

Total = $607

The invoice will therefore contain:

Subtotal:       $550.00
Tax:             $77.00
Discount:        $20.00
-----------------------
TOTAL:          $607.00

Database

The project uses SQLite.

The database file is:

data/invoices.db

SQLite is useful for this project because it is:

Lightweight
Built into Python
Serverless
Easy to deploy
Stored as a single file
Suitable for a small local application

Database Structure

The application uses two main tables.

`invoices`

Stores general invoice information.

invoices
│
├── id
├── invoice_number
├── date
├── customer_name
├── customer_email
├── tax_rate
├── discount
├── subtotal
├── tax
└── total

`invoice_items`

Stores the individual items belonging to an invoice.

invoice_items
│
├── id
├── invoice_id
├── name
├── quantity
├── unit_price
└── total

The relationship looks like:

invoices
   │
   │ 1
   │
   │
   │ many
   ▼
invoice_items

One invoice can contain many invoice items.

For example:

Invoice INV-0001
│
├── Website
├── Hosting
├── Domain
└── Maintenance

PDF Generation

PDF files are generated using the reportlab package.

Generated invoices are saved inside:

invoices/

Example:

invoices/
├── INV-0001.pdf
├── INV-0002.pdf
└── INV-0003.pdf

A generated PDF contains information such as:

INVOICE

Invoice Number: INV-0001
Date: 2026-09-27

Customer:
John Smith
john@example.com

-----------------------------------------------
Item / Service       Quantity   Price   Total
-----------------------------------------------
Website                  1       $500    $500
Hosting                  1        $50     $50
-----------------------------------------------

Subtotal:                         $550
Tax (14%):                         $77
Discount:                          $20
-----------------------------------------------
TOTAL:                            $607

Invoice Management

The application provides several operations.

Create Invoice

Creates a new invoice and:

Calculates the totals
Saves the invoice to SQLite
Saves its items
Generates a PDF

List Invoices

Displays previously created invoices.

Example:

Invoice           Date           Customer                Total
-----------------------------------------------------------------
INV-0003          2026-09-27     Sarah                    $450.00
INV-0002          2026-09-26     Ahmed                    $720.00
INV-0001          2026-09-25     John                     $607.00

View Invoice

Enter an invoice number:

Invoice number: INV-0001

The application retrieves the invoice and its items from SQLite and displays the complete invoice.

Delete Invoice

Enter an invoice number:

Invoice number: INV-0001

The application shows the invoice information and asks for confirmation:

Are you sure you want to delete it? (y/n):

Enter:

to delete the invoice.

Input Validation

The application validates user input where appropriate.

For example, entering:

Quantity: abc

produces:

Invalid number. Please try again.

Required text fields cannot be left empty.

For example:

Customer name:

produces:

This field cannot be empty.

Negative values are also rejected for values such as:

Quantity
Unit price
Tax
Discount

Python Concepts Used

This project demonstrates several important Python concepts.

Functions

The application is divided into functions:

def create_new_invoice():
    ...

def list_invoices():
    ...

def view_invoice():
    ...

Dictionaries

Invoice information can be represented using dictionaries:

invoice = {
    "invoice_number": "INV-0001",
    "customer_name": "John",
    "customer_email": "john@example.com",
    "subtotal": 500,
    "tax": 70,
    "total": 570
}

Dataclasses

The project also provides structured models through models.py:

@dataclass
class InvoiceItem:
    name: str
    quantity: float
    unit_price: float

SQLite

The project uses Python's built-in:

sqlite3

module to communicate with the database.

Parameterized queries are used when inserting user data:

cursor.execute(
    """
    INSERT INTO invoices (...)
    VALUES (?, ?, ?, ...)
    """,
    values
)

This avoids constructing SQL queries by directly concatenating user input.

File System Management

The project uses:

pathlib.Path

to manage directories and files.

Example:

BASE_DIR = Path(__file__).resolve().parent

Exception Handling

The application uses:

try:
    ...
except Exception:
    ...

to handle errors without unexpectedly terminating the application.

Dependencies

The project currently uses:

reportlab

Install it with:

pip install reportlab

Python's standard library provides the other functionality used by the project, including:

sqlite3
datetime
pathlib
dataclasses
typing

Example Workflow

A complete invoice creation workflow looks like this:

1. Start application
        │
        ▼
2. Initialize SQLite database
        │
        ▼
3. Select "Create new invoice"
        │
        ▼
4. Enter customer information
        │
        ▼
5. Add invoice items
        │
        ▼
6. Enter tax and discount
        │
        ▼
7. Calculate invoice
        │
        ├───────────────┐
        ▼               ▼
8. Save database     9. Generate PDF
        │               │
        ▼               ▼
invoices.db          INV-0001.pdf

Example Project Session

============================================================
                   INVOICE GENERATOR
============================================================

1. Create new invoice
2. List invoices
3. View invoice
4. Delete invoice
5. Exit

Choose an option: 1


============================================================
                    CREATE NEW INVOICE
============================================================

--- Customer Information ---

Customer name: John Smith
Customer email: john@example.com

--- Invoice Items ---

Add Item

Item/service name: Website Development
Quantity: 1
Unit price: 500

Add another item? (y/n): y

Add Item

Item/service name: Hosting
Quantity: 1
Unit price: 50

Add another item? (y/n): n

--- Tax & Discount ---

Tax (%): 14
Discount: 20


============================================================
                        INVOICE
============================================================

Invoice number: INV-0001
Date:           2026-09-27
Customer:       John Smith
Email:          john@example.com

------------------------------------------------------------
ITEMS
------------------------------------------------------------
Website Development              1.00 x $    500.00 = $    500.00
Hosting                          1.00 x $     50.00 = $     50.00

------------------------------------------------------------
Subtotal:                                      $    550.00
Tax (14%):                                     $     77.00
Discount:                                      -$     20.00
------------------------------------------------------------
TOTAL:                                         $    607.00
============================================================

Invoice created successfully!

Invoice number: INV-0001
PDF: invoices/INV-0001.pdf

Future Improvements

Possible future versions of the project can add:

Invoice Features

Sequential invoice numbering
Invoice editing
Duplicate invoice
Invoice search
Invoice filtering
Invoice status
Due dates
Payment status
Notes
Terms and conditions

Customer Management

Customer database
Customer history
Customer search
Customer addresses
Customer phone numbers
Customer company names

Business Information

Company name
Company logo
Company address
Phone number
Email
Website
Tax registration number

Currency

Support for multiple currencies:

USD
EUR
GBP
EGP
SAR
AED

PDF Improvements

Company logo
Custom colors
Better typography
Professional invoice templates
Page numbers
Payment instructions
QR codes
Bank information
Signature area

Application Improvements

Graphical user interface
Web interface
REST API
User authentication
Multiple businesses
Cloud database
Online invoice sharing
Email invoices to customers

Planned Architecture

A larger version of the application could eventually look like:

invoice-generator/
│
├── app/
│   │
│   ├── models/
│   │   ├── invoice.py
│   │   ├── customer.py
│   │   └── item.py
│   │
│   ├── services/
│   │   ├── invoice_service.py
│   │   ├── pdf_service.py
│   │   └── customer_service.py
│   │
│   ├── database/
│   │   ├── connection.py
│   │   └── repositories.py
│   │
│   └── utils/
│       ├── validators.py
│       └── formatters.py
│
├── data/
│   └── invoices.db
│
├── invoices/
│
├── tests/
│
├── main.py
├── requirements.txt
└── README.md

This structure would make it easier to expand the project into a larger application.

Testing

Run the application:

python main.py

Test the following:

Create

1. Create new invoice

Create an invoice containing multiple items.

List

2. List invoices

Verify that the newly created invoice appears.

View

3. View invoice

Enter its invoice number and verify that all information is displayed correctly.

Delete

4. Delete invoice

Delete a test invoice and verify that it no longer appears in the invoice list.

PDF

Check:

invoices/

and verify that the corresponding PDF was generated.

Error Handling

Common issues include:

ReportLab is not installed

Error:

ModuleNotFoundError: No module named 'reportlab'

Install it:

pip install reportlab

Database problems

Delete the existing database:

data/invoices.db

Then run the application again.

The application will recreate the database and its tables.

Data Storage

The application stores invoice information locally.

Database:

data/invoices.db

Generated documents:

invoices/

The SQLite database should be backed up if the invoices are important.

Security Considerations

The application is currently designed as a local application.

If it is later converted into a web application, additional security should be implemented, including:

Authentication
Authorization
Password hashing
Input validation
CSRF protection
Secure session handling
Database access controls
File upload validation
Secure PDF storage
HTTPS

License

This project can be licensed according to the requirements of the project owner.

For an open-source project, a common option is the MIT License.

Author

Deadcode

Dead

Name		Name	Last commit message	Last commit date
data		data
invoices		invoices
LICENSE		LICENSE
README.md		README.md
database.py		database.py
invoice.py		invoice.py
main.py		main.py
models.py		models.py
pdf_generator.py		pdf_generator.py
requirements.txt		requirements.txt

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Invoice Generator

Features

Project Structure

File Responsibilities

main.py

invoice.py

models.py

database.py

pdf_generator.py

How the Application Works

Requirements

Installation

1. Clone the repository

2. Create a virtual environment

3. Activate the virtual environment

Linux/macOS

Windows

4. Install dependencies

Running the Application

Creating an Invoice

Invoice Calculation

Database

Database Structure

invoices

invoice_items

PDF Generation

Invoice Management

Create Invoice

List Invoices

View Invoice

Delete Invoice

Input Validation

Python Concepts Used

Functions

Dictionaries

Dataclasses

SQLite

File System Management

Exception Handling

Dependencies

Example Workflow

Example Project Session

Future Improvements

Invoice Features

Customer Management

Business Information

Currency

PDF Improvements

Application Improvements

Planned Architecture

Testing

Create

List

View

Delete

PDF

Error Handling

ReportLab is not installed

Database problems

Data Storage

Security Considerations

License

Author

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`main.py`

`invoice.py`

`models.py`

`database.py`

`pdf_generator.py`

`invoices`

`invoice_items`