Running Your App
You've written the code. Now you need to actually start the server.
In Codespaces (the easy way)
Open the terminal in VS Code and run:
flask run --debugCodespaces will detect the running server and offer to forward the port. Click Open in Browser.
What those words mean
flask— the command-line tool that comes with the Flask libraryrun— start the development server--debug— enable auto-reload. When you save a file, the server restarts automatically. Always use this during development.
The terminal will show:
* Running on http://127.0.0.1:5000127.0.0.1 is localhost — it means "this computer." Your Flask server is running locally. In Codespaces, port forwarding handles this transparently.
The server keeps running
This is different from a normal Python script. A script runs and finishes. A Flask server runs and waits — it's listening for requests. You'll see the terminal "hang" with no new prompt. That's normal. It's working.
To stop the server: Ctrl+C in the terminal.
The alternative: app.run()
You might also see this pattern at the bottom of app.py:
if __name__ == "__main__":
app.run(debug=True)This lets you run the app with python app.py instead of flask run --debug. Same result, different command. The starter code includes this so both approaches work.
When things go wrong
flask: command not found — Flask isn't installed. In Codespaces this shouldn't happen (it's pre-installed). Locally, you'd need pip install flask.
The page shows an error — Read the terminal output. Flask error messages are verbose but helpful. The trick: your mistake might be on line 12, but the crash might show up at line 346 inside some Flask internal file. Look for your filename in the traceback.
Your error messages will sometimes point deep into Flask's own code. Don't panic. Scroll through the traceback and find the line from your file — that's where the actual problem is.