I have a huge project written in PHP and the data is stored in MySQL (mostly MyISAM tables and a bit of InnoDB). Because the project gets bigger and more complicated I want to rewrite it using object oriented programming and in Python. What choice of database engines do I have in Python? What DBs are mostly used for large Python projects (ca. 500 simultaneous and active users on the website)?
I think PostgreSQL is most common in the python world with MySQL close behind. However, it doesn't really matter - the python database api is the same with only minor differences between database engines (mainly the placeholder for values - it might be ? or %s etc.).
However, consider using SQLAlchemy - either just for SQL abstraction or its ORM which is pretty awesome. And either way, unless you want to use database-engine-specific things you can use any database - be it sqlite, mysql, pgsql, oracle or anything else supported by SQLAlchemy.
Even though you might be using something completely different, just in case you use flask as your web framework (you do plan to use a WSGI framework, right?!), there is Flask-SQLAlchemy which makes it extremely easy to get started with it.
In case you are going to use Django it already comes with its own ORM.