3.1 Why you need to know this
You do not need to write code, but you do need to know what you are asking for. In 2.5, Agent decided the page, the formula, the data and the tools on its own. If you know the parts of a web app, you can decide each one yourself and say it in your prompt. That is the difference between an app that runs and an app that does what you wanted.
3.2 Web apps and native apps
A web app runs in a browser and is opened with a link, on any laptop or phone. A native app is installed on one operating system, such as an iPhone app from the App Store. Today you build a web app, so anyone with your link can use it.
3.3 The three parts of a web app
| Part | In plain words | In today’s app | What to tell Copilot |
|---|---|---|---|
| Frontend | What you see and click | Title, input boxes, result, chart | What is on the page, in what order, how it looks |
| Backend | The work done out of sight | Python in app.py: formulas, rules, your secret key |
How each result is calculated, what to do with bad input |
| Data | Where the numbers come from | What you type, then the Sectors API (Module 5) | Which source to use, and which fields |
Most web apps keep the frontend and backend in separate programs. Streamlit is a framework, a ready-made starter kit of code, that lets one Python file do both: each line of Python becomes a piece of the page. That is why one file, app.py, is enough today.
3.4 Local or live?
On your laptop the app is local: Streamlit runs it on your own computer, at the address localhost:8501, and only you can open it. When you deploy it in Module 9, it runs on a server, a computer in a data centre that stays on all day, and it is live: anyone can open its link.
3.5 Mini glossary
| Term | In one plain sentence |
|---|---|
| IDE | A code editor with extra tools; VS Code is one |
| AI agent | An AI that plans and takes several steps on its own |
| Frontend / backend | What you see and click / the work done out of sight |
| API | A set way for one program to ask another for data (Module 5) |
| Server | A computer in a data centre that runs your app all day |
| Deploy | Putting your app on a server so it is live |
| Framework | A starter kit of code; today, Streamlit |
| Bug | The app doing something wrong |