[twunde]

1) What you really want is a mental model of what the application is doing as a whole and what the subsection you're working on now is doing. Look for existing diagrams if possible. If there aren't any start making them as you go along and then have other devs verify. Then try to own a subsection of the code and really understand what it's doing. Once you feel comfortable with that move on to another, preferably related piece.

2)While you normally don't need or want an IDE for dynamic languages, it sounds like this codebase has reached the point where you should be using one and in particular be using the jump to definition shortcut. If you make a type error, this should help you catch it quickly. PyCharm is a good one but there are alternatives.

3) Operational complexity should be owned by someone already. If not, this may be the place where you can make the biggest impact. See if there is centralized logging set up. Make sure the setup/installation and deployment procedures are up to date. These can typically be automated pretty easily

4) An alternative approach is to look at the data first. Once you understand the core data models, you typically understand the application