Build a Budget Tracker App

• ✓Use Python's core datatypes, control flow, and data structures to build working, real logic
• ✓Design a class hierarchy with real encapsulation, inheritance, abstraction, and polymorphism
• ✓Structure a growing script into a real package of modules
• ✓Raise and handle custom exceptions that carry real meaning, not just prevent crashes
• ✓Persist application state to a file and recover gracefully from a missing or corrupted one
• ✓Build a working Streamlit UI and understand why a rerun-based framework needs session_state to hold onto anything
• ✓Run a slow operation on a background thread without freezing the interface, and without the thread ever touching the UI
• ✓Handle exceptions correctly in a Streamlit app's main script and inside a background thread — two genuinely different mechanisms, not one

Build a real, deployable application from first principles in Python — not four disconnected topics, but one tool that gets more capable, one lesson at a time, that you can actually keep using and share a link to.