Streamlit apps are built as simple Python scripts that execute from top to bottom each time the app is run or updated. This execution model means that every line of code is rerun whenever you interact with a widget or when the script file changes. You do not need to define a main function or manage a special event loop: Streamlit handles all of this for you, making it easy to prototype and build interactive dashboards quickly. The order in which you write your code determines the order in which interface elements appear on the page, and any variables or data loaded at the top of your script are re-evaluated on each run.

Widgets such as st.button are interactive elements that can trigger a rerun of the entire script when their state changes. For example, clicking a button causes Streamlit to execute the entire script again from the top, updating the interface according to any changes in widget values or variables. Streamlit manages the state of widgets automatically, so you do not need to write extra code to remember whether a button was clicked or a value was entered. This approach simplifies the development process and ensures that your dashboard always reflects the current state of your code and user inputs.