Flask Application

Running Flask Development Server

Flask comes bundled with a builtin development server for fast local development and testing. This is an easy way to take biweeklybudget for a spin, but carries some important and critical warnings if you use it with real data. For upstream documentation, see the Flask Development Server docs. Please note that the development server is a single process and defaults to single-threaded, and is only realistically usable by one user.

  1. First, setup your environment per Getting Started - Setup.
  2. export FLASK_APP="biweeklybudget.flaskapp.app"
  3. If you’re running against an existing database, see important information in the “Database Migrations” section, below.
  4. flask --help for information on usage:
  • Run App: flask run
  • Run with debug/reload: flask rundev

To run the app against the acceptance test database, use: DB_CONNSTRING='mysql+pymysql://budgetTester@' flask run

By default, Flask will only bind to localhost. If you want to bind to all interfaces, you can add --host= to the flask run commands. Please be aware of the implications of this (see “Security”, below).

If you wish to run the flask app in a multi-process/thread/worker WSGI container, be sure that you run the initdb entrypoint before starting the workers. Otherwise, it’s likely that all workers will attempt to create the database tables or run migrations at the same time, and fail.

Database Migrations

If you run the Flask application (whether in the flask development server or a separate WSGI container) against an existing database and there are unapplied Alembic database migrations, it’s very likely that multiple threads or processes will attempt to perform the same migrations at the same time, and leave the database in an inconsistent and unusable state. As such, there are two important warnings:

  1. Always be sure that you have a recent database backup before upgrading.
  2. You must manually trigger database migrations before starting Flask. This can be done by running the initdb console script provided by the biweeklybudget package (bin/initdb in your virtualenv).


This code hasn’t been audited. It might have SQL injection vulnerabilities in it. It might dump your bank account details in HTML comments. Anything is possible!

To put it succinctly, this was written to be used by me, and me only. It was written with the assumption that anyone who can possibly access any of the application at all, whether in a browser or locally, is authorized to view and/or edit anything and everything related to the application (configuration, everything in the database, everything in Vault if it’s being used). If you even think about making this accessible to anything other than localhost on a computer you physically own, it’s entirely up to you how you secure it, but make sure you do it really well.


By default, the Flask application’s logs go to STDOUT. The BIWEEKLYBUDGET_LOG_FILE environment variable can be set to the absolute path of a file, to cause Flask application logs to go to the file in addition to STDOUT.

MySQL Connection Errors

See Getting Started - MySQL Connection Errors for some information on handling MySQL connection errors.