Invoice Generator

This project demonstrate how to build a simple flask app to generate pdf invoices using flask, flask-sqlalchemy, bootstrap and weasyprint. A common and pure python way to create PDFs is using Reportlab. The problem with Reportlab is that it can be quite time consuming to get the PDFs' design as you want it to be. In this project I'm going to weasyprint to create PDF invoice from a HTML template.

Getting Started

Installing Dependencies

Python 3.7

Follow the instructions to install the latest version of python for your platform in the python docs

Virtual Enviornment

I recommend working within a virtual environment whenever using Python for projects. This keeps your dependencies for each project separate and organaized. Instructions for setting up a virual enviornment for your platform can be found in the python docs

PIP Dependencies

Once you have your virtual environment setup and running, install dependencies by navigating to the project directory and running:

pip install -r requirements.txt

This will install all of the required packages we selected within the requirements.txt file.

Key Dependencies

• Flask is a lightweight backend microservices framework. Flask is required to handle requests and responses.

• SQLAlchemy is the Python SQL toolkit and ORM we'll use handle the lightweight sqlite database. You'll primarily work in app.py and can reference models.py.

• WeasyPrint is a smart solution helping web developers to create PDF documents. In this project it turns a simple HTML page into a PDF invoice.

Running the server

From within the project directory first ensure you are working using your created virtual environment. All environment variables are set within the .flaskenv file:

• Setting the FLASK_ENV variable to development will detect file changes and restart the server automatically.
• Setting the FLASK_APP variable to app.py from the project directory will tell the terminal the application to work with.

To run the server, execute in the terminal from within the proect directory:

flask run

Models

• Positions: Entries represent the individual Positions for the PDF invoices.
• InvoiceDetails: Entry represents the invoice details like client information and invoicing period.

Endpoints

Index

GET '/'

• Returns the index.html template with jinja2 template inheritance of the main.html template.

PositionsForm, Positions Table ad InvoiceForm

GET '/invoice_data'

• Returns the invoice_form.html template with jinja2 template inheritance of the main.html template.
• On the first run of the application it just renders the PoitionsForm.
• If there are entries in Positions, it renders them the positions table and renders the InvoiceForm.
• If there is an entry in InvoiceDetails, it prefills the InvoiceForm.

Add Position

POST '/invoice_data'
PositionsForm submission

• Does a server side form validation of PositionsForm via flask-wtf.
• Adds the new position to the database via the class function insert().

Create Invoice

POST '/invoice_data'
InvoiceForm submission

• Does a server side form validation of InvoiceForm via flask-wtf.
• Adds the provided invoice details to the database via the class function insert() if no entry is in InvoiceDetails.
• If an entry exists it overwrites it via the class funtion update(**kwargs)
• Note: There will only be one entry in InvoiceDetails.

Delete Positions

DELETE '/position_delete'

• Deletes a certain position from Positions specified by the id in the request body.

Request body:

{
    "position_id": "<position id>"
}

Response data:

{
    "url": "<url invoice form endpoint (GET)>"
}

Create Invoice PDF

POST '/invoice_pdf'

• Queries for the previously added data in Positions and InvoiceDetails to fill the invoice template.
• Passes the rendered template as a string to weasyprints' HTML and serves the PDF as an in-memory object via flasks' send_file.
• BytesIO is used, because it is neccessary to hand an object to send_file which has a read() method.