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1. Practice. It’s gotten easier for me over the years because my day job involves regular code/architecture review over tons of repos.

2. Ask the developers.

3. Play with the software and trace through the source code. For flask, make a simple application and grep for the definitions of the functions/methods/classes you’re referencing. While doing this, you will find references to other flask functions. Find read those definitions Ad-infinitum and eventually it’ll “click”.